Raspy, not one becoming agile (4)
I believe the answer is:
spry
'agile' is the definition.
('spry' can be a synonym of 'agile')
'raspy not one becoming' is the wordplay.
'not' is a deletion indicator.
'one' becomes 'a'.
'becoming' indicates an anagram.
'raspy' with 'a' taken away is 'rspy'.
'rspy' with letters rearranged gives 'SPRY'.
(Other definitions for spry that I've seen before include "Still able to move lightly and fast" , "Nimble, brisk" , "Light on one's feet" , "Far from doddering" , "Sprightly" .)