Raider or intruder (7)
I believe the answer is:
invader
I believe this is a double definition.
'raider' is the first definition.
(thesaurus)
'intruder' is the second definition.
(I know that invader is a type of intruder)
(Other definitions for invader that I've seen before include "One attacks" , "Encroacher" , "Attacker from outside" , "One entering as an enemy" , "Military intruder" .)