Head off skirmish — hurry! (4)
I believe the answer is:
rush
'hurry' is the definition.
('rush' can be a synonym of 'hurry')
'head off skirmish' is the wordplay.
'head off' means to remove the first letter.
'skirmish' becomes 'brush' ('brush' can be a synonym of 'skirmish').
'brush' with its initial letter removed is 'RUSH'.
(Other definitions for rush that I've seen before include "High" , "plant developing on shore" , "Drug-induced euphoria" , "Hurry - marsh plant" , "Marsh plant - fly" .)