Stone provided by old chum (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'by old chum' is the wordplay.
'by' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chum' becomes 'pal' (both can mean a friend).
'o'+'pal' is 'OPAL'.
'provided' is the link.
(Other definitions for opal that I've seen before include "Milky-white gemstone with flashes of colour" , "Translucent gemstone" , "Milky-white mineral" , "Semi-precious stone of milky or bluish colour" , "precious jewel" .)