Rebel's first support for attack (4)
I believe the answer is:
raid
'attack' is the definition.
(raid is a kind of attack)
'rebel's first support' is the wordplay.
'first' indicates taking the first letters.
'support' becomes 'aid' ('aid' can be a synonym of 'support'**).
The initial letter of 'rebels' is 'r'.
'r'+'aid'='RAID'
'for' acts as a link.
(Other definitions for raid that I've seen before include "Invade" , "Sally" , "bust" , "Surprise police visit" , "Search by police without warning" .)