Farm worker needing exercises with restrictive back (6)
I believe the answer is:
reaper
'farm worker' is the definition.
(reaper is a kind of farm worker)
'exercises with restrictive back' is the wordplay.
'exercises' becomes 'pe'.
'with restrictive' means one lot of letters goes inside another (I've seen this in other clues).
'back' becomes 'rear' ('rear' can be a synonym of 'back').
'pe' placed into 'rear' is 'REAPER'.
'needing' is the link.
(Other definitions for reaper that I've seen before include "that you don't want to meet if it's grim" , "Farm machine (possibly grim?)" , "Death is described as a grim one" , "Farm worker" , "Farm machinery" .)