Stone in ring friend's put on (4)
I believe the answer is:
opal
'stone' is the definition.
(thesaurus)
'ring friend's put on' is the wordplay.
'ring' becomes 'o'.
'friend' becomes 'pal' (pal is a kind of friend).
'put on' says to put letters next to each other.
'o'+'pal'='OPAL'
'in' is the link.
(Other definitions for opal that I've seen before include "Lustrous stone" , "Delicately coloured gem" , "often semi-transparent" , "Mineral, usually iridescent" , "precious jewel" .)