Power to restrict horrible din late in the day (8)
I believe the answer is:
midnight
'late in the day' is the definition.
'power to restrict horrible din' is the wordplay.
'power' becomes 'might' ('might' can be a synonym of 'power'**).
'to restrict' indicates putting letters inside.
'horrible' indicates an anagram.
'din' with letters rearranged gives 'idn'.
'might' placed around 'idn' is 'MIDNIGHT'.
(Other definitions for midnight that I've seen before include "Macbeth's 'secret, black and --- hags'" , "12am" , "Pitch darkness" , "'It came upon the . . . . . . . . clear' (carol)" , "Twelve hours after noon" .)