Agile agent suppressing resistance (4)
I believe the answer is:
spry
'agile' is the definition.
(synonyms)
'agent suppressing resistance' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'suppressing' indicates putting letters inside.
'resistance' becomes 'r' (electronics abbreviation).
'spy' going around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Quick and agile" , "Light on one's feet" , "Nimble (for one's age?)" , "(Of an old person) lively" , "Sprightly" .)