Roman-style mansion (5)
I believe the answer is:
manor
'mansion' is the definition.
(manor is a kind of mansion)
'roman-style' is the wordplay.
'style' indicates anagramming the letters (style can mean to shape or design).
'roman' anagrammed gives 'MANOR'.
(Other definitions for manor that I've seen before include "house on estate" , "Large country house for Roman" , "Country seat" , "Mansion of a wealthy person" , "local copper's area" .)