It's a stone ring, chum (4)
I believe the answer is:
opal
'it's a stone' is the definition.
(gemstone)
'ring chum' is the wordplay.
'ring' becomes 'O' (O is the shape of a ring).
'chum' becomes 'pal' (both can mean a friend).
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Semi-precious stone of milky or bluish colour" , "National gemstone of Australia" , "Birthstone for October" , "Variegated gemstone" , "Many-coloured gem" .)