Stable mixed up dates close to Derby (6)
I believe the answer is:
steady
'stable' is the definition.
('steady' can be similar in meaning to 'stable')
'mixed up dates close to derby' is the wordplay.
'mixed up' indicates anagramming the letters.
'close to' indicates one should take the final letters (the close/ending of).
The final letter of 'derby' is 'y'.
'dates' with letters rearranged gives 'stead'.
'stead'+'y'='STEADY'
(Other definitions for steady that I've seen before include "Regular, constant, unvarying" , "date seen regularly" , "admirer" , "Balanced, not shaky" , "Maybe 'staid'" .)