Nimble agent nabbing traitor finally (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'agent nabbing traitor finally' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'nabbing' is an insertion indicator.
'finally' suggests the final letters.
The last letter of 'traitor' is 'r'.
'spy' enclosing 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Lively (especially of an older person)" , "Light on one's feet" , "Agile (for one's age?)" , "(Of an old person) lively" , "Energetic" .)