A gemstone for a girl (5)
I believe the answer is:
beryl
This clue is a double definition.
'a gemstone' is the first definition.
(beryl is a kind of gemstone)
'girl' is the second definition.
'for a' acts as a link.
(Other definitions for beryl that I've seen before include "Transparent mineral used as a gemstone" , "Girl's name - gemstone" , "- - Bainbridge (lit.)" , "- Bainbridge, English novelist" , "Emerald or aquamarine" .)