Counteract rotten gang (6)
I believe the answer is:
offset
'counteract' is the definition.
(I know that counteract is a more specific form of offset)
'rotten gang' is the wordplay.
'rotten' becomes 'off' (rotten food is 'off').
'gang' becomes 'set' (set of people).
'off'+'set'='OFFSET'
(Other definitions for offset that I've seen before include "Compensate(d) for" , "Something that compensates" , "Compensation - printing process" , "Place at an angle to the main alignment" , "Counteract to compensate" .)