Being stony, a friend has nothing (4)
I believe the answer is:
opal
'being stony a' is the definition.
I can't judge whether this definition defines the answer.
'friend has nothing' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'has' says to put letters next to each other.
'nothing' becomes 'O' (looks like zero - 0).
'pal' after 'o' is 'OPAL'.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Milky stuff that's hard" , "Gem with delicate colours" , "Mineral used as a gemstone, of hydrated silica" , "Semi-transparent gemstone" , "it may be highly valued" .)