Mate right to stop devil (6)
I believe the answer is:
friend
'mate right' is the definition.
'friend' can be an answer for 'mate' (mate can mean a friend). I'm not certain of the 'right' bit.
'to stop devil' is the wordplay.
'to' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'stop' indicates putting letters inside (stopping can mean blocking a gap).
'devil' becomes 'fiend' (synonyms).
'r' placed inside 'fiend' is 'FRIEND'.
Can you help me to learn more?
(Other definitions for friend that I've seen before include "Member of a religious community" , "Quaker - pal" , "allowed to pass by the guard" , "Organisation's supporter" , "Buddy, chum" .)