Agent keeps Rex nimble (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'agent keeps rex' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'keeps' indicates putting letters inside.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'spy' placed around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Moving quickly and lightly" , "(Of an old person) lively" , "'Agile, nimble (4)'" , "Energetic" , "Nimble, brisk" .)