Roman's date is not a bit small (5)
I believe the answer is:
nones
'roman's date' is the definition.
(type of day in the Roman calendar)
'not a bit small' is the wordplay.
'not a bit' becomes 'none' (none of something).
'small' becomes 's' (abbreviation - e.g. clothes size).
'none'+'s'='NONES'
'is' is the link.
(Other definitions for nones that I've seen before include "part of office" , "Roman Catholic service read about 3pm" , "Day in Roman calendar, nine days before ides" , "In the Roman calendar, the ninth day before the ides" , "Roman monthly date" .)