Are going back in time (3)
I believe the answer is:
era
'time' is the definition.
(I know that era is a type of geologic time)
'are going back' is the wordplay.
'going back' is a reversal indicator.
'are' in reverse letter order is 'ERA'.
'in' acts as a link.
(Other definitions for era that I've seen before include "Eon" , "Geological age" , "Long period of time" , "A particular period of time" , "Characteristic period" .)