Stone in ring presented to mate (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'ring presented to mate' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'presented to' is a charade indicator (letters next to each other).
'mate' becomes 'pal' (I've seen this before).
'o'+'pal'='OPAL'
'in' acts as a link.
(Other definitions for opal that I've seen before include "Milk-coloured gemstone" , "National gemstone of Australia" , "One in a ring perhaps" , "Iridescent quartz-like stone" , "little jewel" .)