Aunt cooked fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'aunt cooked' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'aunt' is an anagram of 'TUNA'.
(Other definitions for tuna that I've seen before include "swimmer" , "Large tasty fish, fond of a nut" , "Fish; edible cactus" , "Large sea-fish" , "Skipjack --, blue-fin --" .)