Entertainer is treat when plastered (7)
I believe the answer is:
artiste
'entertainer' is the definition.
(artiste is a kind of entertainer)
'is treat when plastered' is the wordplay.
'when plastered' indicates an anagram (letters are drunk (plastered) and disorderly).
'is'+'treat'='istreat'
'istreat' is an anagram of 'ARTISTE'.
(Other definitions for artiste that I've seen before include "'Public performer, dancer or singer (7)'" , "Person playing" , "nightclub entertainer?" , "Entertainer, singer or dancer" , "Theatre performer" .)