Valuable rock (3)
I believe the answer is:
gem
'valuable rock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gem that I've seen before include "Valuable stone" , "Emerald or sapphire, say" , "A jewel of an Orangeman" , "Pleasant and cheerful (from German)" , "Half of 7 [SIGN OF THE ZODIAC]" .)