Twelve notes about rings (4)
I believe the answer is:
noon
'twelve' is the definition.
(twelve noon)
'notes about rings' is the wordplay.
'notes' becomes 'nn' (an abbreviation for note written twice).
'about' is an insertion indicator.
'rings' becomes 'oo' (two O's - O resembles a ring).
'nn' placed around 'oo' is 'NOON'.
(Other definitions for noon that I've seen before include "A moment of time" , "12 o'clock" , "when PM starts" , "The middle of the day" , "High time of day?" .)