Some Aztec nobles rose up in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(once upon a time)
'some aztec nobles rose up' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'up' is a reversal indicator (in down clue: letters go upwards).
'ecno' is hidden in the letters of 'aztec nobles rose'.
'ecno' reversed gives 'ONCE'.
(Other definitions for once that I've seen before include "At some former time" , "Just as a single occurrence" , "A single time" , "In the old days" , "In times past" .)