Treated chesty bushwhacker (6)
I believe the answer is:
scythe
'bushwhacker' is the definition.
(blade for cutting corn or bushes)
'treated chesty' is the wordplay.
'treated' indicates anagramming the letters.
'chesty' is an anagram of 'SCYTHE'.
(Other definitions for scythe that I've seen before include "Implement for cutting long grass" , "Manual hay-cutter wielded with both hands" , "Cutting instrument" , "Reaper's cutter" , "Manual grass cutter" .)