After revolution, exist a long time (3)
I believe the answer is:
era
'a long time' is the definition.
(a long period of time)
'after revolution exist' is the wordplay.
'after revolution' says the letters should be written backwards (letters revolve back).
'exist' becomes 'are' ('they exist' means 'they are').
'are' written backwards gives 'ERA'.
(Other definitions for era that I've seen before include "Geological period" , "8" , "Series of years" , "generation, maybe" , "Cycle" .)