Constant backer interrupting game (10)
I believe the answer is:
changeless
'constant' is the definition.
(synonyms)
'backer interrupting game' is the wordplay.
'backer' becomes 'angel' (synonyms).
'interrupting' indicates putting letters inside.
'game' becomes 'chess' (chess is a kind of game).
'angel' going within 'chess' is 'CHANGELESS'.
(Other definitions for changeless that I've seen before include "Consistent" , "Very steady" , "same as always" .)