Space for transubstantiation, not the front of church (4)
I believe the answer is:
apse
'church' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for apse that I've seen before include "where praetor sat" , "Strange peas in the church recess" , "Primate holds saint to be a" , "Architectural feature in a church" , "Recess with domed rof in a church" .)