Infiltrator; burglar (8)
I believe the answer is:
intruder
I believe this is a double definition.
'infiltrator' is the first definition.
(thesaurus)
'burglar' is the second definition.
(thesaurus)
(Other definitions for intruder that I've seen before include "One making uninvited entry" , "One entering unasked" , "Burglar, or one who barges in" , "Trespasser" , "Interloper" .)