Rogue swimmer in sport (7)
I believe the answer is:
curling
'sport' is the definition.
(curling is a sport)
'rogue swimmer' is the wordplay.
'rogue' becomes 'cur' (both can mean a despicable person).
'swimmer' becomes 'ling' (ling is a type of fish).
'cur'+'ling'='CURLING'
'in' acts as a link.
(Other definitions for curling that I've seen before include "providing treatment for shock?" , "Not a straightforward" , "Game played on ice" , "Winter sport where stones are slid towards a target" , "Doing the twist" .)