Uninvited visitor (8)
I believe the answer is:
intruder
'uninvited visitor' 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 "Trespasser" , "Interloper" , "Butter in" , "Person who enters uninvited, perhaps a burglar" , "Unwanted visitor" .)