A green gemstone (7)
I believe the answer is:
emerald
'a green gemstone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for emerald that I've seen before include "...re medal? (Anag)" , "Small size of type, in printing" , "Highly-valued variety of beryl" , "On of the Greens" , "Precious gemstone" .)