Champion, as a cause (7)
I believe the answer is:
espouse
'champion as a cause' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for espouse that I've seen before include "Marry -- take on as a cause" , "help!" , "Adopt - give support to" , "Follow and promote a cause" , "See opus (anag)" .)