So, one of the people I am writing about in my thesis has two main areas of work (a and b). I'm just focusing on one of those areas (a) as that's the one that's relevant to my topic. However, the other area (b) isn't totally irrelevant and I've been bringing in bits and pieces to support my conclusions drawn from material in the area that I am focusing on. If that makes sense?!
I want to use concept 'x' which my guy has written about in area (b) in my discussion of area (a). Trouble is, unlike on the other occasions when I have referred to area (b), this time concept 'x' opens a whole can of worms. It is derived from another theorist whose work I've only the smallest knowledge of, and it raises loads of questions which I just can't answer within the scope of what I'm doing.
I feel really bad that I haven't researched the background to the idea more thoroughly - it was something I kept meaning to do but kept putting off as not the most relevant (or interesting!) bit. Now I'm out of time really as I'm writing up and my sup wants to see this work... well... yesterday.
Has anyone else had an experience where you want to use an idea but you don't have time or space in the thesis to go through all the ins and outs of it? - you just want to say, look that's there in the background and enables me to make this point, moving on...
I feel like I'm in deep trouble, like this is a big and unfillable hole and that my work just isn't up to scratch if I can still not know my sh*t at this stage! That's the main worry - why don't I know all about this? And what do I do as there isn't really time to fix it?
PANIC!

Don't panic. I hit a similar snag and ended up having a meltdown in front of my supervisor, who gently pointed out that the flaws in my work were always going to be much more obvious to me than to anyone else. My suggestion would be to skate over it as best you can in the chapter, but to consider including the issue/concept/theorist/new research question in the conclusion as a fruitful angle for further research. If you can find a review article of the theorist's work you could also stick a footnote in saying see x for a fuller discussion of this point. Then when you've submitted do some reading and get a more coherent account together of where this new theorist / concept connects with your thesis in case it comes up in your viva. Worst case scenario it's set as a correction in which case you've already done the work. Of course this will only work if your supervisor is prioritising submission over perfection.

My thesis is so multi-disciplinary that I can't possibly include everything about some of the areas which are linked to my arguments. For example I'm touching on aspects of Medieval memory theory but I can't go into great detail about this. (In fact one of my colleagues has done her whole thesis on memory). So I am taking out the bit that is relevant to what I am saying about it but then I hope that I will be able to read up on it more widely so that in my viva (if I ever get to that stage), I can show that I do know the background to the ideas I am using.
So I would say don't worry about trying to explain the can of worms but be aware that they are there and the reasons why you haven't gone down that road for your arguments.

Thank you so much for those replies. Feel much better just to have a different perspective on things.
About to go and kick start the work now (have got into this weird 11-8 working pattern) - the focus on submission rather than perfection thing is really good. I know my sup is focused on submission and he will tell me to leave this if I ask him. So I should just leave it and, as you say, do the background reading before the viva.
Thanks!

that's kinda my approach as well Florence but I'm a little worried that I am starting to build up a list of things I have 'gen' up on before the viva :-(.