Sonorous ring ring rings beginning to toll (7)
I believe the answer is:
orotund
'sonorous' is the definition.
(thesaurus)
'ring ring rings beginning to toll' is the wordplay.
'ring' becomes 'o'.
'ring' becomes 'round' ('round' can be a synonym of 'ring'**).
'rings' is an insertion indicator.
'beginning to' indicates taking the first letters.
The initial letter of 'toll' is 't'.
'round' going around 't' is 'rotund'.
'o'+'rotund'='OROTUND'
(Other definitions for orotund that I've seen before include "Not dour (anag.)" , "Pompous - grandiose" , "booming" , "Pompous (in style)" , "Of speech, dignified and pompous" .)