Collapsed round part of church (4)
I believe the answer is:
apse
'church' is the definition.
(I've seen this before)
'collapsed round part' is the wordplay.
'part' says the answer is hidden in the clue.
'APSE' is hidden in the letters of 'collapsed round'.
'of' acts as a link.
(Other definitions for apse that I've seen before include "place in church" , "Semicircular end of a church, behind the altar" , "Where you could find a magistrate's chair" , "Recess at eastern end of a church" , "Primate holds saint to be a" .)