Volunteers in bar with medium fowl (6)
I believe the answer is:
bantam
'fowl' is the definition.
(bantam is a kind of fowl)
'volunteers in bar with medium' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'in' is an insertion indicator.
'bar' becomes 'ban' (bar can mean to ban or prohibit).
'with' says to put letters next to each other.
'medium' becomes 'm' (as in UK clothing sizes).
'ban'+'m'='banm'
'ta' placed into 'banm' is 'BANTAM'.
(Other definitions for bantam that I've seen before include "Breed of domestic fowl" , "Nab mat (anag)" , "Aggressive person" , "Boxing weight or feisty fowl" , "poultry" .)