Diamonds get cooler (3,3)
I believe the answer is:
ice bag
'cooler' is the definition.
(I've seen this before)
'diamonds get' is the wordplay.
'diamonds' becomes 'ice' (slang term for diamonds).
'get' becomes 'bag' (bag can mean to secure or obtain).
'ice'+'bag'='ICE-BAG'
(Other definitions for ice bag that I've seen before include "cooler" , "cold compress" , "Waterproof container" , "Container for diamonds, or cold cubes to stop swelling" , "Cold item applied to reduce swelling" .)