Brace secures centre of Rome cathedral (5)
I believe the answer is:
duomo
'cathedral' is the definition.
(I've seen this before)
'brace secures centre of rome' is the wordplay.
'brace' becomes 'duo' ('duo' can be a synonym of 'brace').
'secures' is an insertion indicator.
'centre of' says to take the centre.
The central letters of 'rome' are 'om'.
'duo' enclosing 'om' is 'DUOMO'.
(Other definitions for duomo that I've seen before include "Roman church" , "Italian cathedral" , "foreign cathedral" , "Large church" .)