Vicious to stop one being supportive (6)
I believe the answer is:
onside
'supportive' is the definition.
'vicious to stop one' is the wordplay.
'vicious' becomes 'sid' (Sid Vicious).
'to stop' is an insertion indicator.
'sid' going inside 'one' is 'ONSIDE'.
'being' is the link.
(Other definitions for onside that I've seen before include "is in state to continue playing" , "In a legal position (football)" , "in a permitted position in sport" , "in a position to score?" , "Working towards the same goal" .)