A fellow broker (5)
I believe the answer is:
agent
'broker' is the definition.
(synonyms)
'a fellow' is the wordplay.
'fellow' becomes 'gent' (synonyms).
'a'+'gent'='AGENT'
(Other definitions for agent that I've seen before include "Catalyst" , "Middleman" , "Representative - means" , "One employed to incite others to commit illegal acts" , "One who acts for another" .)