Part of church primate to embrace Sabbath (4)
I believe the answer is:
apse
'part of church' is the definition.
('apse' is part of 'church')
'primate to embrace sabbath' is the wordplay.
'primate' becomes 'ape' (I've seen this before).
'to embrace' is an insertion indicator.
'sabbath' becomes 's'.
'ape' enclosing 's' is 'APSE'.
(Other definitions for apse that I've seen before include "part of cathedral" , "space for altar" , "Church recess in lapsed condition" , "Recess in church wall" , "Part of a church with the altar" .)