Girl with a heart of stone (6)
I believe the answer is:
cherry
This could be a double definition.
'girl' is the first definition.
'heart of stone' is the second definition.
The definition and answer can be both plants as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'with a' acts as a link.
Can you help me to learn more?
(Other definitions for cherry that I've seen before include "Red fruit with a single hard stone" , "The colour of a red fruit" , "Greyfriars boy identified" , "Virginity (slang)" , "cheer? No longer" .)