Respectable chap joins swimmer (7)
I believe the answer is:
genteel
'respectable' is the definition.
(thesaurus)
'chap joins swimmer' is the wordplay.
'chap' becomes 'gent' (I've seen this in other clues**).
'joins' means one lot of letters go next to another.
'swimmer' becomes 'eel' (eel is a kind of fish).
'gent'+'eel'='GENTEEL'
(Other definitions for genteel that I've seen before include "Courtly, cultivated" , "Cultured; polite" , "Courteous, well-bred" , "Polite and well-bred" , "Artificially polite" .)