Nobleman's land, could be Roman (5)
I believe the answer is:
manor
'nobleman's land' is the definition.
(I've seen this before)
'could be roman' is the wordplay.
'could be' indicates an anagram.
'roman' with letters rearranged gives 'MANOR'.
(Other definitions for manor that I've seen before include "local copper's area" , "police area" , "Roman (anag.)" , "Police district (slang)" , "Operational base" .)