Unwelcome guest (8)
I believe the answer is:
intruder
'unwelcome guest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for intruder that I've seen before include "Person who comes in being neither invited nor wanted" , "Housebreaker" , "Gatecrasher" , "Trespasser" , "Uninvited and unwelcome visitor" .)