Big fish aunt cooked (4)
I believe the answer is:
tuna
'big fish' is the definition.
(tuna are big fish)
'aunt cooked' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'aunt' anagrammed gives 'TUNA'.
(Other definitions for tuna that I've seen before include "Large salt water fish" , "Type of fish" , "Fish; edible cactus" , "Sea creature" , "Tunny" .)