Help brought in by band that is attacking (7)
I believe the answer is:
raiding
'is attacking' is the definition.
(I've seen this before)
'help brought in by band' is the wordplay.
'help' becomes 'aid' (synonyms).
'brought in by' is an insertion indicator.
'band' becomes 'ring' (a ring is a band of metal).
'aid' placed within 'ring' is 'RAIDING'.
'that' acts as a link.
(Other definitions for raiding that I've seen before include "Attacking" , "Plundering" , "Making inroads" .)