Cockney conceals date in Rome (4)
I believe the answer is:
ides
I believe this is a double definition.
'cockney conceals' is the first definition.
('hides' - 'hide' can be a synonym of 'conceal' - said with a cockney accent, dropping the 'H')
'date in rome' is the second definition.
(Roman calendar term, as in the Ides of March)
(Other definitions for ides that I've seen before include "Caesar's fateful day" , "Side of March when Caesar died" , "Time of March when Caesar was assassinated" , "Fateful day for Caesar" , "Mid-month day in the Roman calendar" .)