I had to back the Gunners to attack (4)
I believe the answer is:
raid
'attack' is the definition.
(raid is a kind of attack)
'i had to back the gunners' is the wordplay.
'i had' becomes 'id' (I'd).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'back' shows that the letters should be reversed in order.
'the gunners' becomes 'ar' (I can't justify this - if you can you should believe this answer much more).
'ar' back-to-front is 'ra'.
'id' put after 'ra' is 'RAID'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for raid that I've seen before include "Maraud" , "Inroad" , "Invasion" , "Smash-and-grab?" , "Rifle" .)