Ring friend to get a stone (4)
I believe the answer is:
opal
'a stone' is the definition.
(gemstone)
'ring friend' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'to get' is the link.
(Other definitions for opal that I've seen before include "Translucent gem" , "Semi-precious stone of milky or bluish colour" , "often semi-transparent" , "Milk-coloured gemstone" , "Translucent gemstone" .)