Happen to note there’s a ringer going about! (6)
I believe the answer is:
befall
'happen to' is the definition.
(synonyms)
'note there's a ringer going about' is the wordplay.
'note' becomes 'f' (musical note).
'there's' means one lot of letters go next to another.
'ringer' becomes 'bell' (a bell rings).
'going about' indicates putting letters inside.
'f'+'a'='fa'
'fa' placed into 'bell' is 'BEFALL'.
(Other definitions for befall that I've seen before include "Turn out" , "Happen to (someone)" , "Happen, occur" .)