Nimble agent? About right (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'agent? about right' is the wordplay.
'agent?' becomes 'spy' (spy is a kind of agent).
'about' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'spy' placed around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Far from doddering" , "Still able to move lightly and fast" , "Agile (for one's age?)" , "Nimble, brisk" , "'Agile, nimble (4)'" .)