Very big sign describing one's bar (9)
I believe the answer is:
ostracise
'bar' is the definition.
(I've seen this before)
'very big sign describing one's' is the wordplay.
'very big' becomes 'os' (abbreviation for outsize).
'sign' becomes 'trace' ().
'describing' is an insertion indicator (describe can mean to form the outline of).
'one's' becomes 'i's' (Roman numeral).
'trace' enclosing 'is' is 'tracise'.
'os'+'tracise'='OSTRACISE'
(Other definitions for ostracise that I've seen before include "Banish from group" , "Drive out" , "Cast out, send to Coventry" , "Shun, exclude" , "Socrates and I are made to exclude from society" .)