Nimble undercover agent runs inside (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'undercover agent runs inside' is the wordplay.
'undercover agent' becomes 'spy' ('spy' can be a synonym of 'undercover agent').
'runs' becomes 'r' (cricket abbreviation).
'inside' means one lot of letters goes inside another.
'spy' going around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Energetic" , "Light on one's feet" , "Moving quickly and lightly" , "(Of an old person) lively" , "'Active and lively, nimble (4)'" .)