Regularity is a quality of nightfall, perhaps (8)
I believe the answer is:
evenness
'regularity' is the definition.
(I know that evenness is a type of regularity)
'quality of nightfall perhaps' is the wordplay.
I cannot really see how this works, but
'quality' could be 'ness' (I've seen this in other clues) and 'ness' is found within the answer.
'nightfall' could be 'eve' (I've seen this in another clue) and 'eve' is found in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'is a' is the link.
Can you help me to learn more?
(Other definitions for evenness that I've seen before include "Uniformity, smoothness" , "Smooth quality" , "Quality of the uniform" , "great calm" , "Levelness" .)