Sense fellow swimmer (4)
I believe the answer is:
feel
'sense' is the definition.
(I know that sense can be written as feel)
'fellow swimmer' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'swimmer' becomes 'eel' (eels swim).
'f'+'eel'='FEEL'
(Other definitions for feel that I've seen before include "Experience, perceive" , "Have a belief; touch" , "Be aware of - texture" , "Sense or touch" , "Have sensation of" .)