Vicious perhaps to stop one being supportive (6)
I believe the answer is:
onside
'supportive' is the definition.
'vicious perhaps to stop one' is the wordplay.
'vicious perhaps' becomes 'Sid' (Sid Vicious).
'to stop' indicates putting letters inside (stopping can mean blocking a gap).
'sid' put into 'one' is 'ONSIDE'.
'being' acts as a link.
(Other definitions for onside that I've seen before include "In agreement (colloq.)" , "in position to proceed?" , "Legally positioned at football etc" , "is in state to continue playing" , "legitimate" .)