Stupid upset over silly thing when day begins (8)
I believe the answer is:
midnight
'day begins' is the definition.
I can't tell whether this definition defines the answer.
'stupid upset over silly thing' is the wordplay.
'stupid' becomes 'dim' (similar in meaning).
'upset over' says the letters should be written backwards (knocked over).
'silly' indicates an anagram.
'dim' written backwards gives 'mid'.
'thing' anagrammed gives 'night'.
'mid'+'night'='MIDNIGHT'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for midnight that I've seen before include "Twelve, perhaps" , "possibly go to sleep this time?" , "when PM finishes" , "When PM turns to AM" , "Twelve hours after noon" .)