Harvester extremely prone to break back (6)
I believe the answer is:
reaper
'harvester' is the definition.
(I know that harvester can be written as reaper)
'extremely prone to break back' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'to break' indicates putting letters inside (inserted letters break into the word).
'back' becomes 'rear' (something's rear is its back).
'prone' with its centre taken out is 'pe'.
'pe' placed within 'rear' is 'REAPER'.
(Other definitions for reaper that I've seen before include "Farm machinery" , "One harvesting crop" , "Harvester of, eg, 4 dn" , "Death is described as a grim one" , "that you don't want to meet if it's grim" .)