Unwelcome guest is, in time, less polite (8)
I believe the answer is:
intruder
'unwelcome guest' is the definition.
(I've seen this before)
'in time less polite' is the wordplay.
'time' becomes 't' (abbreviation).
'less polite' becomes 'ruder' ('rude' can be the opposite of 'polite' so perhaps 'ruder' is reasonable for 'less polite').
'in'+'t'+'ruder'='INTRUDER'
'is' acts as a link.
(Other definitions for intruder that I've seen before include "Uninvited person like burglar" , "Unwelcome guest" , "Housebreaker" , "Unwanted visitor" , "Butter in" .)