Casserole staple (4)
I believe the answer is:
tuna
'casserole staple' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tuna that I've seen before include "Fish, often tinned" , "Large edible sea fish, often canned" , "Common food fish" , "Marine food fish" , "Large food fish" .)