A precious stone (7)
I believe the answer is:
'a precious stone' is the definition.
(I know that emerald is a type of precious stone)
This is the entire clue.
(Other definitions for emerald that I've seen before include "Gem; type of moth" , "Mere lad (anag.)" , "...re medal? (Anag)" , "Green variety of beryl" , "Gem associated with Ireland" .)