Opposed to gang's leader, or getting on? (5)
I believe the answer is:
aging
'getting on?' is the definition.
(get on can mean to get older)
'opposed to gang's leader' is the wordplay.
'opposed to' becomes 'agin' (dialect form of against).
'leader' indicates taking the first letters.
The initial letter of 'gangs' is 'g'.
'agin'+'g'='AGING'
'or' is the link.
(Other definitions for aging that I've seen before include "Maturing" , "Getting old" , "Growing older" , "Getting older" , "Becoming more grown-up" .)