Run into agent being active (4)
I believe the answer is:
spry
'active' is the definition.
(similar in meaning)
'run into agent' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'into' indicates putting letters inside.
'agent' becomes 'spy' (spy is a kind of agent).
'r' going into 'spy' is 'SPRY'.
'being' is the link.
(Other definitions for spry that I've seen before include "Still able to move lightly and fast" , "Fleet" , "(Of an old person) lively" , "Nimble, brisk" , "Quick and agile" .)