Bounder supporting Bury, an uninvited guest (10)
I believe the answer is:
interloper
'an uninvited guest' is the definition.
(an interloper is an intruder or invader)
'bounder supporting bury' is the wordplay.
'bounder' becomes 'loper' (I've seen this in another clue).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'bury' becomes 'inter' (to inter is to put into the ground).
'loper' after 'inter' is 'INTERLOPER'.
(Other definitions for interloper that I've seen before include "Uninvited attendee" , "one entering uninvited" , "Intruder" , "one who gatecrashes" .)