Plead on behalf of another (9)
I believe the answer is:
intercede
'plead on behalf of another' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for intercede that I've seen before include "Plead in favour of" , "step in" , "Intervene, on behalf of another" , "Come between as mediator" , "Act as peacemaker between two parties" .)