Still during key part of the day (7)
I believe the answer is:
evening
'part of the day' is the definition.
(I know that evening is part of dark)
'still during key' is the wordplay.
I cannot quite understand how this works, but
'still' could be 'even' (synonyms) and 'even' is found within the answer.
'during' could be 'in' (e.g. during February means in February) and 'in' is found within the answer.
'key' could be 'g' (musical key) and 'g' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for evening that I've seen before include "Hours before bedtime" , "after dark?" , "Ironing" , "Latter part of the day" , "End part of the day" .)