Couple on second cathedral (5)
I believe the answer is:
duomo
'cathedral' is the definition.
(I've seen this before)
'couple on second' is the wordplay.
'couple' becomes 'duo' (synonyms).
'on' means one lot of letters go next to another.
'second' becomes 'mo' (both are short periods of time).
'duo'+'mo'='DUOMO'
(Other definitions for duomo that I've seen before include "foreign cathedral" , "Roman church" , "Italian cathedral" , "Large church" .)