A jewel ring is returned by letter (5)
I believe the answer is:
omega
'letter' is the definition.
(I know that omega is a type of letter)
'a jewel ring is returned' is the wordplay.
'jewel' becomes 'gem' (synonyms).
'ring' becomes 'o' (letter resembling a ring).
'is returned' is a reversal indicator.
'a'+'gem'+'o'='agemo'
'agemo' backwards is 'OMEGA'.
'by' acts as a link.
(Other definitions for omega that I've seen before include "Traditional story" , "Last thing" , "Last (Gk. letter)" , "Art of story-telling by body movement" , "End of sequence" .)