Stone old chum (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'old chum' is the wordplay.
'old' becomes 'o' (abbreviation).
'chum' becomes 'pal' (both can mean a friend).
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Milky iridescent stone" , "raw material" , "Translucent gem" , "Semi-precious stone of b;uish white colour" , "Iridescent quartz-like stone" .)