What one eats in this assembly? (4)
I believe the answer is:
diet
This clue is a double definition.
'what one eats' is the first definition.
(I've seen this before)
'this assembly?' is the second definition.
(diet is a kind of assembly)
'in' is the link.
(Other definitions for diet that I've seen before include "(Perhaps, try to) reduce rump; parliament" , "cut down" , "Restrict food-intake" , "See 19" , "food one eats on a regular basis" .)