Announcement: 50% off diamonds! (3)
I believe the answer is:
ice
'diamonds' is the definition.
(slang term for diamonds)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ice that I've seen before include "Cold demeanour" , "Decorate with frosting" , "dessert" , "Cold stuff used in enticement" , "Cooler" .)