Attacker's assistant grabbed by bishop (6)
I believe the answer is:
raider
'attacker's' is the definition.
(thesaurus)
'assistant grabbed by bishop' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'grabbed by' means one lot of letters goes inside another.
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'aide' put within 'rr' is 'RAIDER'.
(Other definitions for raider that I've seen before include "Armed insurgent" , "Marauder" , "One making incursion" , "Surprise attacker" , "One plundering" .)