Aunt mad for fish (4)
I believe the answer is:
tuna
'for fish' is the definition.
(tuna is a type of fish)
'aunt mad' is the wordplay.
'mad' indicates an anagram.
'aunt' anagrammed gives 'TUNA'.
(Other definitions for tuna that I've seen before include "Species of fish" , "Large salt water fish" , "Fish; edible cactus" , "Popular food fish" , "Fish sold in cans" .)