Loud swimmer’s stroke (4)
I believe the answer is:
feel
'stroke' is the definition.
(thesaurus)
'loud swimmer's' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'swimmer' becomes 'eel' (eels swim).
'f'+'eel'='FEEL'
(Other definitions for feel that I've seen before include "Sense or touch" , "Experience, perceive" , "Be aware of - texture" , "Have sensation of" , "Consider" .)