When the contract is made, they're better than diamonds (6)
I believe the answer is:
hearts
'diamonds' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Maybe they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for hearts that I've seen before include "Scottish footballers" , "Body pumps" , "Cores" , "Scots club nickname; suit" , "Vital organs in the pack" .)