Fish in a casserole (4)
I believe the answer is:
tuna
'fish in a casserole' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tuna that I've seen before include "Large tasty fish, fond of a nut" , "Large edible sea fish, often canned" , "Species of fish" , "Warm-water food fish" , "Large fish for the strange aunt" .)