In time, not as polite as unwanted guest (8)
I believe the answer is:
intruder
'unwanted guest' is the definition.
(someone intruding on an event)
'in time not as polite' is the wordplay.
'time' becomes 't' (abbreviation).
'not as polite' becomes 'ruder' ('rude' can be the opposite of 'polite' so perhaps 'ruder' is reasonable for 'not as polite').
'in'+'t'+'ruder'='INTRUDER'
'as' is the link.
(Other definitions for intruder that I've seen before include "Burglar, or one who barges in" , "Unwelcome guest" , "Uninvited person like burglar" , "Person who comes in being neither invited nor wanted" , "Gatecrasher" .)