Unusual Roman dwelling house (5)
I believe the answer is:
manor
'dwelling house' is the definition.
(I know that manor is a type of mansion)
'unusual roman' is the wordplay.
'unusual' indicates an anagram.
'roman' anagrammed gives 'MANOR'.
(Other definitions for manor that I've seen before include "Grand but strange Roman house" , "house on estate" , "Big house - police district" , "lord's home" , "police area" .)