Nimble agent crossing river (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'agent crossing river' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'spy' going around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Nimble, brisk" , "Moving quickly and lightly" , "Nimble (for one's age?)" , "Energetic" , "'Agile, nimble (4)'" .)