Collapsed around part of cathedral (4)
I believe the answer is:
apse
'part of cathedral' is the definition.
(I've seen this before)
'collapsed around' is the wordplay.
'around' says the answer is hidden in the clue.
'APSE' can be found hidden inside 'collapsed'.
(Other definitions for apse that I've seen before include "Large recess in church" , "Offshoot from the choir, nave or aisle in a church" , "Peas (anag) - found in church" , "Strange peas in the church recess" , "Recess at east end of church" .)