Left darkness extremely quickly (9)
I believe the answer is:
overnight
'quickly' is the definition.
(eg something might change overnight)
'left darkness' is the wordplay.
'left' becomes 'over' (I've seen this before).
'darkness' becomes 'night' (night is a period of darkness).
'over'+'night'='OVERNIGHT'
'extremely' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for overnight that I've seen before include "Extending from dusk to dawn" , "Until morning; suddenly" , "sudden" , "from one day to the next" , "until tomorrow" .)