Dish's undercooked part (7)
I believe the answer is:
rarebit
'dish's' is the definition.
(rarebit is a kind of dish)
'undercooked part' is the wordplay.
'undercooked' becomes 'rare' (I've seen this before).
'part' becomes 'bit' (both mean a piece of something).
'rare'+'bit'='RAREBIT'
(Other definitions for rarebit that I've seen before include "Cheese on toast - arbiter (anag)" , "Welsh snack" , "Arbiter of tasty Welsh dish of cheese on toast" , "Cheesy snack" , "Arbiter of Welsh dish with melted cheese on toast" .)