Fish aunt prepared (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a kind of fish)
'aunt prepared' is the wordplay.
'prepared' is an anagram indicator.
'aunt' anagrammed gives 'TUNA'.
(Other definitions for tuna that I've seen before include "Large edible sea fish, often canned" , "Yellow-finned fish" , "Marine food fish" , "Large fish for the strange aunt" , "Widely-eaten fish" .)