Agile agent capturing traitor finally (4)
I believe the answer is:
spry
'agile' is the definition.
(synonyms)
'agent capturing traitor finally' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'capturing' means one lot of letters goes inside another.
'finally' says to take 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 "'Agile, nimble (4)'" , "Sprightly" , "Energetic" , "'Active and lively, nimble (4)'" , "Lively (especially of an older person)" .)