Cut diamonds upon diamonds (4)
I believe the answer is:
dice
'cut' is the definition.
(dicing is a kind of cutting)
'diamonds upon diamonds' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'upon' says to put letters next to each other.
'diamonds' becomes 'ice' (I've seen this before).
'd'+'ice'='DICE'
(Other definitions for dice that I've seen before include "Cut into small cubes" , "Make into chips" , "Random number generators" , "Take a risk" , "Gambling pieces" .)