It's what's in the pie that gets stuck in your teeth (7)
I believe the answer is:
filling
'pie that gets stuck in your teeth' is the definition.
The definition and answer can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'it's what's in' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for filling that I've seen before include "Tooth insert" , "Requirement of teeth or petrol tank?" , "Satisfying (meal); one given by dentist" , "Contents" , "Finding someone for (a post)" .)