Do something as a favour (6)
I believe the answer is:
oblige
'do something as a favour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for oblige that I've seen before include "Perform a service for (someone)" , "Hold in debt" , "Perform a favour for" , "Coerce or accommodate" , "Do something as a favour to (someone)" .)