Primate must welcome son for recess (4)
I believe the answer is:
apse
'for recess' is the definition.
(apse is a kind of recess)
'primate must welcome son' is the wordplay.
'primate' becomes 'ape' (large primate).
'must welcome' is an insertion indicator.
'son' becomes 's' (genealogical abbreviation for son).
'ape' enclosing 's' is 'APSE'.
(Other definitions for apse that I've seen before include "Semicircular recess in church" , "Primate holds saint to be a" , "Recess at east end of church" , "Recess with domed rof in a church" , "Semicircular end of a church, behind the altar" .)