Unwanted guest less polite after putting away first unit of beer (8)
I believe the answer is:
intruder
'unwanted guest' is the definition.
(someone intruding on an event)
'less polite after putting away first unit of beer' is the wordplay.
'less polite' becomes 'ruder' ('rude' can be the opposite of 'polite' so perhaps 'ruder' is reasonable for 'less polite').
'after' is a charade indicator (letters next to each other).
'putting away first' means to remove the first letter.
'unit of beer' becomes 'pint' (I've seen this before).
'pint' with its first letter taken off is 'int'.
'ruder' after 'int' is 'INTRUDER'.
(Other definitions for intruder that I've seen before include "Unwelcome sort" , "Unwanted visitor" , "Trespasser" , "Unwelcome guest" , "One entering uninvited" .)