Old English, 'sweet implement (7)
I believe the answer is:
execute
'implement' is the definition.
(executing is a kind of implementing)
'old english 'sweet' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'english' becomes 'E' (abbreviation).
''sweet' becomes 'cute' (cute can mean sweet or adorable).
'ex'+'e'+'cute'='EXECUTE'
(Other definitions for execute that I've seen before include "Discharge" , "shoot" , "Guillotine" , "Put to death - put into effect" , "Put (plan) into effect; kill (a condemned person)" .)