Active for one's age (4)
I believe the answer is:
spry
'active for one's age' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spry that I've seen before include "Light on one's feet" , "Still able to move lightly and fast" , "'Active and lively, nimble (4)'" , "Quick and agile" , "Agile (for one's age?)" .)