Are in the ascendancy for a time (3)
I believe the answer is:
era
'time' is the definition.
(era is a kind of time)
'are in the ascendancy' is the wordplay.
'in the ascendancy' says the letters should be written backwards (in a down clue, letters go up or ascend).
'are' reversed gives 'ERA'.
'for a' is the link.
(Other definitions for era that I've seen before include "Series of years" , "8" , "Geological period" , "Are involved in time span" , "Special time" .)