A fellow negotiator (5)

I believe the answer is:

agent

'negotiator' is the definition.
(agent is a kind of negotiator)

'a fellow' is the wordplay.
'fellow' becomes 'gent' (I've seen this before**).
'a'+'gent'='AGENT'

(Other definitions for agent that I've seen before include "One's representative" , "Operative" , "Delegate" , "Person acting on behalf of another" , "Representative - means" .)

I've seen this clue in The Telegraph.

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