Doubly right about an assistant being the attacker (6)
I believe the answer is:
raider
'the attacker' is the definition.
(I've seen this before)
'doubly right about an assistant' is the wordplay.
'doubly' indicates the doubling of some letters.
'right' becomes 'r' (common abbreviation).
'about' means one lot of letters goes inside another.
'an assistant' becomes 'aide' (aide is a kind of assistant).
'r' duplicated is 'rr'.
'rr' enclosing 'aide' is 'RAIDER'.
'being' acts as a link.
(Other definitions for raider that I've seen before include "One making a rapid surprise attack" , "One plundering" , "looter" , "Marauder" , "One making incursion" .)