Unwelcome guests maybe when gate-crashing different parties (9)
I believe the answer is:
parasites
'unwelcome guests' is the definition.
(parasites live off their hosts)
'when gate-crashing different parties' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'gate-crashing' means one lot of letters goes inside another (letters gate-crash into the word).
'different' is an anagram indicator.
'parties' is an anagram of 'parites'.
'as' going into 'parites' is 'PARASITES'.
'maybe' is the link.
(Other definitions for parasites that I've seen before include "unwelcome guests" , "Passengers" , "They live off other organisms" , "they need indulgent hosts" , "Hangers-on" .)