Demon securing King’s mate (6)
I believe the answer is:
friend
'mate' is the definition.
(thesaurus)
'demon securing king's' is the wordplay.
'demon' becomes 'fiend' ('fiend' can be a synonym of 'demon').
'securing' indicates putting letters inside.
'king' becomes 'r' (r is abbreviation for rex).
'fiend' going around 'r' is 'FRIEND'.
(Other definitions for friend that I've seen before include "Close associate" , "Quaker - pal" , "Pal, mate" , "Companion; Quaker" , "One who gives proof of genuine regard" .)