Alert secret agent captures ringleader (4)
I believe the answer is:
spry
'alert' is the definition.
(I've seen this before)
'secret agent captures ringleader' is the wordplay.
'secret agent' becomes 'spy' (spy is a kind of secret agent).
'captures' indicates putting letters inside.
'ringleader' becomes 'r' (first letter of ring).
'spy' placed around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "(Of an old person) lively" , "Nimble (for one's age?)" , "Quick and agile" , "Energetic" , "'Agile, nimble (4)'" .)