Respectable chap joins swimmer (7)
I believe the answer is:
Here is my best explanation:
'respectable' is the definition.
(I've seen this before)
'chap joins swimmer' is the subsidiary indication.
'chap' becomes 'gent' (I've seen this in other clues**).
'joins' is a charade indicator (letters next to each other).
'swimmer' becomes 'eel' (eel is a kind of fish).
(Other definitions for genteel that I've seen before include "Mannerly", "Courteous, well-bred", ""Refayned"", "Affectedly refined and upper-crust", "Politely refined", "Polite and well-bred", "Well-bred", "Well mannered", "Belonging to polite society" and "Suited to polite society".)