Dark having arisen, mysterious thing the witching hour (8)
I believe the answer is:
midnight
'the witching hour' is the definition.
(I've seen this before)
'dark having arisen mysterious thing' is the wordplay.
I cannot quite see how this works, but
'dark' could be 'night' (it's dark at nighttime) and 'night' is found within the answer.
The remaining letters 'mid' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for midnight that I've seen before include "When PM turns to AM" , "Twenty four hundred hours (or 0000) hrs" , "possibly go to sleep this time?" , "-- Sun; -- blue" , "The end of a day" .)