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' indicates a hidden word.
'APSE' is hidden within '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" .)