Nimble secret agent getting right in (4)
I believe the answer is:
spry
'nimble' is the definition.
(I know that nimble can be written as spry)
'secret agent getting right in' is the wordplay.
I cannot really see how this works, but
'right' could be 'r' (abbreviation) and 'r' is found in the answer.
'agent' could be 'spy' (spy is a kind of agent) and 'spy' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for spry that I've seen before include "Agile (for one's age?)" , "Alert" , "Energetic" , "Nimble, brisk" , "'Agile, nimble (4)'" .)