Energetic secret agent captures traitor, finally (4)
I believe the answer is:
spry
'energetic' is the definition.
(I've seen this before)
'secret agent captures traitor finally' is the wordplay.
'secret agent' becomes 'spy' (spy is a kind of secret agent).
'captures' is an insertion indicator.
'finally' says to take the final letters.
The final letter of 'traitor' is 'r'.
'spy' enclosing 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Energetic" , "Still able to move lightly and fast" , "'Active and lively, nimble (4)'" , "Lively (especially of an older person)" , "Moving quickly and lightly" .)