Diamonds in reserve (3)
I believe the answer is:
ice
This clue is a double definition.
'diamonds' is the first definition.
(slang term for diamonds)
'reserve' is the second definition.
(both can mean aloofness)
'in' acts as a link.
(Other definitions for ice that I've seen before include "engine" , "Cold demeanour" , "Diamonds (sl)" , "Slippery surface" , "It's cold and slippy" .)