Attacker one helping to support Resistance (6)
I believe the answer is:
raider
'attacker' is the definition.
(I've seen this before)
'one helping to support resistance' is the wordplay.
'one helping' becomes 'aider' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'resistance' becomes 'r' (abbreviation).
'aider' put after 'r' is 'RAIDER'.
Can you help me to learn more?
(Other definitions for raider that I've seen before include "Surprise attacker" , "Depredator" , "One making incursion" , "person taking food from larder" , "Maker of surprise attack" .)