Sound of a bell (4)
(Other definitions for ding that I've seen before include "-- -a-ling" , "Sound" , "- - dong" , "Keep sounding" .)
'sound of a bell' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for knell that I've seen before include "Awesome ring" , "it precipitates death" , "Toll, ring" , "Toll (bell)" , "Sound of tolling bell" .)
(Other definitions for ding-dong that I've seen before include "Violent exchange - sound of a bell" , "Fight (with hammer and tongs?)" , "Sound of a clapper bell" , "Fierce argument (colloq.)" , "Sound of bell or slang argument" .)