Interloper (8)
I believe the answer is:
intruder
'interloper' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for intruder that I've seen before include "Trespasser" , "Butter in" , "One entering unasked" , "Person who comes in being neither invited nor wanted" , "Uninvited and unwelcome visitor" .)