Revolutionary period? (4)
I believe the answer is:
year
'revolutionary period?' is the definition.
(the Earth completing a revolution around the Sun)
This is all the clue.
(Other definitions for year that I've seen before include "One tenth of a decade" , "Period of 5 across" , "Time taken by a planet to orbit the sun" , "Division of school" , "1% of a century" .)