Gatecrasher (8)
I believe the answer is:
intruder
'gatecrasher' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for intruder that I've seen before include "Housebreaker" , "Unwelcome guest" , "Burglar, or one who barges in" , "Gatecrasher" , "One making uninvited entry" .)