Swimmer outside clubs being vulgar (4)
I believe the answer is:
racy
'vulgar' is the definition.
(thesaurus)
'swimmer outside clubs' is the wordplay.
'swimmer' becomes 'ray' (ray is a kind of aquatic vertebrate).
'outside' means one lot of letters goes inside another.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'ray' placed around 'c' is 'RACY'.
'being' acts as a link.
(Other definitions for racy that I've seen before include "Risqu" , "bawdy quality" , "Slightly indecent" , "Sexually suggestive" , "Spicy" .)