Note diamonds are in cubes (4)
I believe the answer is:
dice
'cubes' is the definition.
(I've seen this in another clue)
'note diamonds' is the wordplay.
'note' becomes 'd' (musical note).
'diamonds' becomes 'ice' (slang term for diamonds).
'd'+'ice'='DICE'
'are in' is the link.
(Other definitions for dice that I've seen before include "Small cube" , "Chop up (food)" , "Gambling items" , "Numbered cube(s)" , "Gaming cube(s)" .)