Primate securing small niche (4)
I believe the answer is:
apse
'niche' is the definition.
(apse is a kind of niche)
'primate securing small' is the wordplay.
'primate' becomes 'ape' (I've seen this before).
'securing' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ape' enclosing 's' is 'APSE'.
(Other definitions for apse that I've seen before include "Niche in church" , "Architectural feature in a church" , "Large recess in church" , "In sacred space" , "Domed section of a church" .)