Compel to do a favour? (6)
I believe the answer is:
oblige
I believe this is a double definition.
'compel' is the first definition.
(I know that compel can be written as oblige)
'do a favour?' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for oblige that I've seen before include "Coerce or accommodate" , "Put oneself out for" , "Morally influence" , "Bind morally or legally" , "Perform a service for (someone)" .)