Hard to move rest around, hard to separate grain (6)
I believe the answer is:
thresh
'separate grain' is the definition.
(I've seen this in another clue)
'hard to move rest around hard' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to move' indicates an anagram (I've seen 'moves' mean this).
'around' is an insertion indicator.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'rest' is an anagram of 'tres'.
'h' going into 'tres' is 'thres'.
'thres'+'h'='THRESH'
'to' is the link.
(Other definitions for thresh that I've seen before include "Beat out grain from husks of corn" , "Separate grain with flail" , "as one may having crop" , "Separate seed from grain" , "Beat (stalks to get grain)" .)