Diamonds in plastic envelope (3)
I believe the answer is:
ice
'diamonds' is the definition.
(slang term for diamonds)
'in plastic envelope' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ICE' is hidden within 'plastic envelope'.
(Other definitions for ice that I've seen before include "Precious stones" , "Rocks on which drink is served?" , "Solid H2O" , "Glaciate" , "Winter formation" .)