Duck, chum! It's a stone! (4)
I believe the answer is:
opal
'it's a stone' is the definition.
(gemstone)
'duck chum' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'chum' becomes 'pal' (both can mean a friend).
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Semiprecious stone of white and bluish colour" , "Precious stone" , "it may be highly valued" , "Milky white gem" , "Mineral, usually iridescent" .)