Ring with a precious stone, returned with a letter (5)
I believe the answer is:
omega
'a letter' is the definition.
(omega is a kind of letter)
'ring with a precious stone returned' is the wordplay.
'ring' becomes 'o' (**).
'with' is a charade indicator (letters next to each other).
'precious stone' becomes 'gem' ('gem' can be a synonym of 'precious stone').
'returned' says the letters should be written backwards.
'a'+'gem'='agem'
'agem' written backwards gives 'mega'.
'o'+'mega'='OMEGA'
'with' acts as a link.
(Other definitions for omega that I've seen before include "in Greece, the limit!" , "Greek long 'o'" , "Foreign character" , "Conclusion" , "completes a series" .)