Nothing, chums, but stones (5)
I believe the answer is:
opals
'stones' is the definition.
(opals are precious gems)
'nothing chums' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'chums' becomes 'pals' ('pal' can be a synonym of 'chum').
'o'+'pals'='OPALS'
'but' is the link.
(Other definitions for opals that I've seen before include "Semi-precious stones of milky or bluish colour" , "jewels" , "Milky-white stones" , "Milky gemstones with delicate colours" , "Milky sparkling gemstones" .)