Regular date in stable (6)
I believe the answer is:
steady
This clue is a double definition.
'regular date' is the first definition.
(I've seen this in another clue)
'stable' is the second definition.
(similar in meaning)
'in' is the link.
(Other definitions for steady that I've seen before include "Firm; regular" , "Changeless, constant" , "Constant - stable" , "Not wavering" , "Reliable - firm" .)