Diamonds to decorate cake? (3)
I believe the answer is:
ice
This clue is a triple definition!
'diamonds' is the first definition.
(slang term for diamonds)
'to decorate' is the second definition.
(to ice a cake is to decorate it)
'cake?' is the third definition.
(I have seen 'Top cake' mean 'ice' so perhaps 'cake' could also mean 'ice')
(Other definitions for ice that I've seen before include "engine" , "Frozen water; kill (slang)" , "Cold demeanour" , "Frozen water - diamonds" , "A state of water" .)