Gem right to fit into ring (5)
I believe the answer is:
pearl
'gem' is the definition.
(I know that pearl is a type of gem)
'right to fit into ring' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to fit into' indicates putting letters inside (I've seen this in other clues).
'ring' becomes 'peal' (pealing is a kind of ringing).
'r' inserted into 'peal' is 'PEARL'.
(Other definitions for pearl that I've seen before include "White - jewel" , "Smooth lustrous round structure of calcium carbonate" , "Precious gem" , "one regularly in stitches" , "coming out of shell?" .)