Old keys (4)
I believe the answer is:
aged
I believe this clue is a double definition.
'old' is the first definition.
(similar in meaning)
'keys' is the second definition.
(A, G, E and D are musical keys)
(Other definitions for aged that I've seen before include "Senior" , "Mature" , "not like a callow youth" , "Old or ancient" , "Ripened, matured" .)