Stone old friend (4)
I believe the answer is:
opal
'stone' is the definition.
(thesaurus)
'old friend' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'pal' (pal is a kind of friend).
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "One's stony" , "Gem with delicate colours" , "Semi-precious stone of milky or bluish colour" , "Milky-white gemstone with flashes of colour" , "Precious stone" .)