Force required to cut up diamonds (6)
I believe the answer is:
police
'force' is the definition.
(police force is a kind of force)
'to cut up diamonds' is the wordplay.
'to cut' becomes 'lop' (lopping is a kind of cutting**).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'diamonds' becomes 'ice' (I've seen this before).
'lop' reversed gives 'pol'.
'pol'+'ice'='POLICE'
'required' is the link.
(Other definitions for police that I've seen before include "Monitor; control" , "Bobby" , "Keep watch on" , "9a 19d" , "Lawmen" .)