Active agent captures Russia's leader (4)
I believe the answer is:
spry
'active' is the definition.
('spry' can be similar in meaning to 'active')
'agent captures russia's leader' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'captures' means one lot of letters goes inside another.
'leader' says to take the initial letters.
The first letter of 'russias' is 'r'.
'spy' going around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Moving quickly and lightly" , "Agile (for one's age?)" , "Nimble, brisk" , "(Of an old person) lively" , "'Agile, nimble (4)'" .)