Part of church collapsed? Only part (4)
I believe the answer is:
apse
'part of church' is the definition.
(part of a church building)
'collapsed? only part' is the wordplay.
'part' says the answer is hidden in the clue.
'APSE' is hidden in the letters of 'collapsed only'.
(Other definitions for apse that I've seen before include "place in church" , "space for altar" , "Recess at east end of church" , "Semicircular end of a church, behind the altar" , "Church recess with domed or arched roof" .)