Affair with female swimmer (5)
I believe the answer is:
fling
'affair' is the definition.
(I have seen 'Brief affair' mean 'fling' so perhaps 'affair' could also mean 'fling')
'with female swimmer' is the wordplay.
'with' means one lot of letters go next to another.
'female' becomes 'f' (abbreviation).
'swimmer' becomes 'ling' (ling is a kind of aquatic vertebrate**).
'f' put next to 'ling' is 'FLING'.
(Other definitions for fling that I've seen before include "Toss" , "Launch or propel" , "good time" , "Throw as in Highland dance?" , "Throw violently" .)