A stone to duck, chum (4)
I believe the answer is:
opal
'a stone' is the definition.
(thesaurus)
'duck chum' is the wordplay.
'duck' becomes 'o'.
'chum' becomes 'pal' ('pal' can be a synonym of 'chum').
'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" .)