Primate holds opening of Sabbath recess (4)
I believe the answer is:
apse
'recess' is the definition.
(apse is a kind of recess)
'primate holds opening of sabbath' is the wordplay.
'primate' becomes 'ape' (large primate).
'holds' is an insertion indicator.
'opening of' suggests taking the first letters.
The initial letter of 'sabbath' is 's'.
'ape' placed around 's' is 'APSE'.
(Other definitions for apse that I've seen before include "Church recess with domed or arched roof" , "Rounded end of building" , "Projection to the east end of a church" , "Offshoot from the choir, nave or aisle in a church" , "Strange peas in the church recess" .)