Ring mate to get the gem (4)
I believe the answer is:
opal
'the gem' is the definition.
(opal is a kind of gem)
'ring mate' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'mate' becomes 'pal' (both can mean a friend).
'o'+'pal'='OPAL'
'to get' is the link.
(Other definitions for opal that I've seen before include "Glassy gemstone" , "Semi-transparent gemstone" , "Translucent gemstone" , "Milky-white gemstone with flashes of colour" , "Delicately-coloured gem" .)