Harvest worker extremely prone to break back (6)
I believe the answer is:
reaper
'harvest worker' is the definition.
(someone reaping crops)
'extremely prone to break back' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'to break' means one lot of letters goes inside another.
'back' becomes 'rear' (synonyms).
'prone' with its centre taken out is 'pe'.
'pe' placed within 'rear' is 'REAPER'.
(Other definitions for reaper that I've seen before include "Farmworker" , "Harvester of, eg, 4 dn" , "Crop harvester" , "being grim when you're late" , "One gathers" .)