Are back for a long time (3)
I believe the answer is:
era
'a long time' is the definition.
(a long period of time)
'are back' is the wordplay.
'back' says the letters should be written in reverse.
'are' reversed gives 'ERA'.
'for' acts as a link.
(Other definitions for era that I've seen before include "Important time" , "Geological age" , "Major division of geological time" , "Period, age" , "8" .)