Respectable fellow swimmer (7)
I believe the answer is:
genteel
'respectable' is the definition.
(genteel means respectable or polished)
'fellow swimmer' is the wordplay.
'fellow' becomes 'gent' (synonyms).
'swimmer' becomes 'eel' (eels swim).
'gent'+'eel'='GENTEEL'
(Other definitions for genteel that I've seen before include "Refined in taste and manners" , "Artificially polite" , "Cultured; polite" , "Courteous, well-bred" , "Terribly polite" .)