G? (8)
I believe the answer is:
midnight
'g?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
This is the entire clue.
(Other definitions for midnight that I've seen before include "Macbeth's 'secret, black and --- hags'" , "type of oil burnt by late work" , "12 hours from noon" , "The moment when the date changes" , "The end of a day" .)