Gatecrasher (8)
I believe the answer is:
intruder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gatecrasher</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for intruder that I've seen before include "Housebreaker" , "Unwelcome guest" , "Burglar, or one who barges in" , "Gatecrasher" , "One making uninvited entry" .)