Ring contains rare gem (5)
I believe the answer is:
pearl
'gem' is the definition.
(I know that pearl is a type of gem)
'ring contains rare' is the wordplay.
'ring' becomes 'peal' (synonyms).
'contains' is an insertion indicator.
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'peal' enclosing 'r' is 'PEARL'.
(Other definitions for pearl that I've seen before include "Precious gem" , "Gem from a bivalve" , "Much loved and valued person" , "Gem; an attacked Harbor" , "Paragon" .)