Playing card, most valuable one? (7)
I believe the answer is:
diamond
This clue is a double definition.
'playing card' is the first definition.
(diamond is a kind of playing card)
'most valuable one?' is the second definition.
(diamond can mean anything valuable)
(Other definitions for diamond that I've seen before include "The hard stuff" , "Ace" , "One of her best friends" , "Gem stone" , "Small type" .)