Old friend has a stone (4)
I believe the answer is:
opal
'a stone' is the definition.
(gemstone)
'old friend' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'has' acts as a link.
(Other definitions for opal that I've seen before include "Rock" , "Birthstone for October" , "Milky-white gemstone with flashes of colour" , "Form of hydrated silica, often used as a gemstone" , "Precious stuff" .)