Fellow constituent behind the bar (11)
I believe the answer is:
counterpart
'fellow' is the definition.
(thesaurus)
'constituent behind the bar' is the wordplay.
'constituent' becomes 'part' (a part is a constituent of something).
'behind' is a charade indicator (letters next to each other).
'the bar' becomes 'counter' (as in the counter of a pub or bar).
'part' after 'counter' is 'COUNTERPART'.
(Other definitions for counterpart that I've seen before include "Each of two copies of a legal document" , "Match" , "See opposite number" , "Duplicate" .)