Stone provided by old chum (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'by old chum' is the wordplay.
'by' says to put 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' put next to 'pal' is 'OPAL'.
'provided' acts as a 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" .)