Are taken back a number of years (3)
I believe the answer is:
era
'a number of years' is the definition.
'are taken back' is the wordplay.
'taken back' says the letters should be written in reverse.
'are' reversed gives 'ERA'.
(Other definitions for era that I've seen before include "Major division of geological time" , "Memorable period" , "8" , "A particular period of time" , "Specific time" .)