Backing for Leonard enthralled by fifty karat ring (5)
I believe the answer is:
knell
'ring' is the definition.
(knelling is a kind of ringing)
'backing for leonard enthralled by fifty karat' is the wordplay.
'backing for' says the letters should be written backwards.
'leonard' becomes 'len' (I've seen this in other clues).
'enthralled by' is an insertion indicator ('enthral' can mean 'capture').
'fifty' becomes 'l' (Roman numeral).
'karat' becomes 'k' (this could be a standard abbreviation I've not previously seen).
'l'+'k'='lk'
'len' going inside 'lk' is 'llenk'.
'llenk' reversed gives 'KNELL'.
Can you help me to learn more?
(Other definitions for knell that I've seen before include "Solemn sound of a bell" , "Awesome ring" , "Toll, ring" , "Temperate" , "(Of bell) ring solemnly" .)