Collapsed within part of church (4)
I believe the answer is:
apse
'part of church' is the definition.
('apse' is part of 'church')
'collapsed within' is the wordplay.
'within' indicates a hidden word.
'APSE' can be found hidden inside 'collapsed'.
(Other definitions for apse that I've seen before include "Rounded end of building" , "Church recess in lapsed condition" , "Offshoot from the choir, nave or aisle in a church" , "Part of a church with the altar" , "Recess at eastern end of a church" .)