Fellow bishop breaks deadlock (10)
I believe the answer is:
stablemate
'fellow' is the definition.
(I've seen this before)
'bishop breaks deadlock' is the wordplay.
'bishop' becomes 'B' (abbreviation used in chess).
'breaks' indicates putting letters inside.
'deadlock' becomes 'stalemate' (synonyms).
'b' going into 'stalemate' is 'STABLEMATE'.
(Other definitions for stablemate that I've seen before include "Partner" , "Nag" , "One from the same organisation as another" , "Maybe fellow Arab" , "person in same outfit" .)