Twelve years on, and zero changes (7)
I believe the answer is:
noonday
'twelve' is the definition.
(twelve o'clock)
'years on and zero changes' is the wordplay.
'years' becomes 'y' (abbreviation).
'zero' becomes 'o'.
'changes' is an anagram indicator.
'y'+'on'+'and'+'o'='yonando'
'yonando' anagrammed gives 'NOONDAY'.
(Other definitions for noonday that I've seen before include "Time when the sun is highest" , "Meridional" , "Midday, 12p.m." , "Twelve a.m." , "12 o'clock" .)