Unusual coin found in food (7)
I believe the answer is:
'food' is the definition.
(rarebit is a kind of food)
'unusual coin' is the wordplay.
'unusual' becomes 'rare' (I've seen this in another clue).
'coin' becomes 'bit' (term for various historical coins).
'found in' acts as a link.
(Other definitions for rarebit that I've seen before include "cheesy meal" , "on Welsh table?" , "See 8 Across" , "Cheese on toast - arbiter (anag)" , "Toasted cheese and bread" .)