A diamond ring, returned in the end (5)
I believe the answer is:
omega
'end' is the definition.
(I've seen this before)
'a diamond ring returned' is the wordplay.
'diamond' becomes 'gem' (I've seen this in other clues).
'ring' becomes 'o'.
'returned' says the letters should be written in reverse.
'a'+'gem'+'o'='agemo'
'agemo' backwards is 'OMEGA'.
'in the' acts as a link.
(Other definitions for omega that I've seen before include "Ending of a series or sequence" , "Art of story-telling by body movement" , "Final outcome" , "symbol of resistance?" , "Foreign character" .)