"Not in a million years!" (5)
I believe the answer is:
never
'not in a million years' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for never that I've seen before include "'It's now or - - ' (14 song)" , "when hell freezes over?" , "Emphatic negative" , "Not ever" , "Under no circumstances" .)