Aunt mad for swimmer (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a type of fish)
'aunt mad' is the wordplay.
'mad' indicates an anagram.
'aunt' is an anagram of 'TUNA'.
'for' acts as a link.
(Other definitions for tuna that I've seen before include "Sea creature" , "Large tasty fish, fond of a nut" , "A fish; sounds like piano juster" , "Large fish for the strange aunt" , "Widely-eaten fish" .)