A stone to duck, chum (4)
I believe the answer is:
opal
'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'
'to' is the link.
(Other definitions for opal that I've seen before include "Rock" , "Many-coloured gem" , "Semiprecious stone of white and bluish colour" , "precious jewel" , "Milky-white mineral" .)