In the church, a postscript to a note (4)
I believe the answer is:
apse
'in the church' is the definition.
(I've seen this before)
'a postscript to a note' is the wordplay.
'postscript' becomes 'ps' (abbreviation).
'to a note' becomes 'e' (musical note).
'a'+'ps'+'e'='APSE'
(Other definitions for apse that I've seen before include "room for an altar" , "Church's part" , "may be seen in church!" , "Large recess in church" , "Strange peas in the church recess" .)