Mediator interrupted Republican, mad he's not up front (6,6)
I believe the answer is:
honest broker
'mediator' is the definition.
(I have seen 'Impartial mediator ' mean 'honest broker' so perhaps 'mediator' could also mean 'honest broker')
'interrupted republican mad he's not up front' is the wordplay.
'interrupted' becomes 'broke' (breaking is a kind of interrupting).
'republican' becomes 'R' (abbreviation in US politics).
'mad' indicates anagramming the letters.
'up front' says to put letters next to each other.
'hes'+'not'='hesnot'
'hesnot' anagrammed gives 'honest'.
'broke'+'r'='broker'
'broker' put after 'honest' is 'HONEST BROKER'.
(Other definitions for honest broker that I've seen before include "member of ACAS?" , "Fair-dealing insurance agent" , "Impartial intermediary" , "Trustworthy intermediary" , "Impartial mediator" .)