Nimble agent set about run (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'agent set about run' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'set about' is an insertion indicator.
'run' becomes 'r' (cricket abbreviation).
'spy' enclosing 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Quick and agile" , "Lively (especially of an older person)" , "Moving quickly and lightly" , "Sprightly" , "(Of an old person) lively" .)