Demon gets right mate (6)
I believe the answer is:
friend
'mate' is the definition.
(I've seen this before)
'demon gets right' is the wordplay.
'demon' becomes 'fiend' (synonyms).
'gets' is an insertion indicator (gets can mean captures or absorbs).
'right' becomes 'r' (common abbreviation).
'fiend' going around 'r' is 'FRIEND'.
(Other definitions for friend that I've seen before include "Companion; Quaker" , "Member of religious society" , "Member of a religious community" , "Pal, amigo" , "one quaking in society?" .)