Are in the ascendant for a time (3)
I believe the answer is:
era
'time' is the definition.
(era is a kind of time)
'are in the ascendant' is the wordplay.
'in the ascendant' says the letters should be written in reverse.
'are' backwards is 'ERA'.
'for a' is the link.
(Other definitions for era that I've seen before include "Series of years" , "Geological period" , "Epoch, period" , "Memorable period" , "Age of importance" .)