Nimble agent catching leader in race (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'agent catching leader in race' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'catching' is an insertion indicator.
'leader in race' becomes 'r' (I've seen this in other clues).
'spy' going around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Agile (for one's age?)" , "Energetic" , "Nimble (for one's age?)" , "Light on one's feet" , "Lively (especially of an older person)" .)