When evening begins, declared champion goes over defeat (9)
I believe the answer is:
nightfall
'when evening' is the definition.
'nightfall' can be an answer for 'evening' (thesaurus). I'm not sure about the 'when' bit.
'begins declared champion goes over defeat' is the wordplay.
I cannot really understand how this works, but
'over' could be 'all' ('all over' can be a synonym of 'over') and 'all' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nightfall that I've seen before include "The onset of darkness" , "when the light fails" , "End of day" , "after sunset" , "Dusk" .)