Go on the offensive (6)
I believe the answer is:
attack
'go on the offensive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for attack that I've seen before include "Strike" , "Ambush - mug - criticise - fit" , "Assault or criticise violently" , "mount an offensive" , "Brownish pigment" .)