Historic rent then due in six hours? (7,3)
I believe the answer is:
quarter day
I believe this clue is a double definition.
'historic rent then due' is the first definition.
(some rent was historically collected every quarter)
'six hours?' is the second definition.
(a quarter of a day)
'in' is the link.
(Other definitions for quarter day that I've seen before include "when payment is due?" , "When to pay the rent" , "during which payments must be made" , "Christmas, say" , "a time to show mercy?" .)