Primate buried saint in church vault (4)
I believe the answer is:
apse
'church vault' is the definition.
'apse' can be an answer for 'vault' (thesaurus). I'm unsure of the 'church' bit.
'primate buried saint' is the wordplay.
'primate' becomes 'ape' (large primate).
'buried' means one lot of letters goes inside another.
'saint' becomes 's'.
'ape' enclosing 's' is 'APSE'.
'in' is the link.
Can you help me to learn more?
(Other definitions for apse that I've seen before include "Architectural feature in a church" , "Church area" , "Recess with domed rof in a church" , "part of cathedral" , "Recess at eastern end of a church" .)