On behalf of (3)
I believe the answer is:
for
'on behalf of' is the definition.
(eg speaking on behalf of or for someone else)
This is all the clue.
(Other definitions for for that I've seen before include "Pro" , "In favour of; since" , "On the side of" , "and 16 and 20: Notwithstanding" , "Favouring" .)