May birthstone (7)
I believe the answer is:
emerald
'may birthstone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for emerald that I've seen before include "Ireland is often called The .......Isle" , "On of the Greens" , "Precious gemstone" , "Green gemstone" , "Gem; type of moth" .)