Active agent arrests Republican (4)
I believe the answer is:
spry
'active' is the definition.
('spry' can be similar in meaning to 'active')
'agent arrests republican' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'arrests' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'spy' going around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Fleet" , "Lively (especially of an older person)" , "Quick and agile" , "Far from doddering" , "(Of an old person) lively" .)