Fine swimmer's instinct (4)
I believe the answer is:
feel
'instinct' is the definition.
(thesaurus)
'fine swimmer's' is the wordplay.
'fine' becomes 'f' (abbreviation).
'swimmer' becomes 'eel' (eel is a kind of fish**).
'f'+'eel'='FEEL'
(Other definitions for feel that I've seen before include "Perceive by touch" , "Have a belief; touch" , "Leg (of mutton/lamb)" , "Experience, perceive" , "16A [JUDGE]" .)