Active agent hiding a ringleader (4)
I believe the answer is:
spry
'active' is the definition.
('spry' can be similar in meaning to 'active')
'agent hiding a ringleader' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'hiding' indicates putting letters inside.
'a ringleader' becomes 'r' (first letter of ring).
'spy' placed around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Agile (for one's age?)" , "Lively (especially of an older person)" , "(Of an old person) lively" , "Still able to move lightly and fast" , "Nimble (for one's age?)" .)