Aristocrat performing behind pub (5)
I believe the answer is:
baron
'aristocrat' is the definition.
(baron is a kind of aristocrat)
'performing behind pub' is the wordplay.
'performing' becomes 'on' (as in being on stage).
'behind' means one lot of letters go next to another.
'pub' becomes 'bar' (both are drinking establishments).
'on' after 'bar' is 'BARON'.
(Other definitions for baron that I've seen before include "Nobleman providing a portion of beef" , "powerful businessman" , "Lowest rank of British nobility" , "A lord; sounds like childless" , "Titled man for a portion of beef" .)