Ring friend for a stone (4)
I believe the answer is:
opal
'a stone' is the definition.
(thesaurus)
'ring friend' is the wordplay.
'ring' becomes 'o'.
'friend' becomes 'pal' (pal is a kind of friend).
'o'+'pal'='OPAL'
'for' is the link.
(Other definitions for opal that I've seen before include "Iridescent gemstone" , "it may be highly valued" , "Precious stone" , "National gemstone of Australia" , "Gem stone" .)