Are back for long time (3)
I believe the answer is:
era
'for long time' is the definition.
(a long period of time)
'are back' is the wordplay.
'back' says the letters should be written backwards.
'are' written backwards gives 'ERA'.
(Other definitions for era that I've seen before include "Characteristic period" , "Major division of geological time" , "Cycle" , "Distinctive period of time" , "Long historical period" .)