Aristocrat in pub - nothing new (5)
I believe the answer is:
'aristocrat' is the definition.
(baron is a kind of aristocrat)
'pub nothing new' is the wordplay.
'pub' becomes 'bar' (I've seen this before).
'nothing' becomes 'o' (looks like zero - 0).
'new' becomes 'n'.
'in' acts as a link.
(Other definitions for baron that I've seen before include "Lowest rank of British peerage" , "Lowest title of peerage" , "Titled man for a portion of beef" , "Member of the nobility" , "powerful businessman" .)