Looter, one helping to support leader of revolt (6)
I believe the answer is:
raider
'looter' is the definition.
(synonyms)
'one helping to support leader of revolt' is the wordplay.
'one helping' becomes 'aider' (someone who aids others).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'leader of' indicates taking the first letters.
The initial letter of 'revolt' is 'r'.
'aider' put after 'r' is 'RAIDER'.
(Other definitions for raider that I've seen before include "One making incursion" , "One making a rapid surprise attack" , "One who makes surprise attack, perhaps to steal" , "Surprise attacker" , "One attacks" .)