Matured sage Derby shows it (4)
I believe the answer is:
aged
'matured' is the definition.
('age' can be a synonym of 'mature')
'sage derby shows it' is the wordplay.
'shows it' says the answer is hidden in the clue.
'AGED' is hidden in the letters of 'sage derby'.
(Other definitions for aged that I've seen before include "Mature" , "not a modern copy" , "not like a callow youth" , "Got older" , "Geriatric" .)