Rebel leader has assistance for attack (4)
I believe the answer is:
raid
'attack' is the definition.
(I know that raid is a type of attack)
'rebel leader has assistance' is the wordplay.
'leader' indicates taking the first letters.
'has' says to put letters next to each other.
'assistance' becomes 'aid' (synonyms).
The initial letter of 'rebel' is 'r'.
'r'+'aid'='RAID'
'for' acts as a link.
(Other definitions for raid that I've seen before include "Surprise police visit" , "Enter a territory and take spoils" , "Go on the attack" , "Assault, attack" , "Invade" .)