In Skye a really revolutionary period (4)
I believe the answer is:
year
'revolutionary period' is the definition.
(the Earth completing a revolution around the Sun)
'in skye a really' is the wordplay.
'in' indicates a hidden word.
'YEAR' is hidden in the letters of 'skye a really'.
(Other definitions for year that I've seen before include "26 fortnights" , "Twelvemonth" , "1% of a century" , "Manage" , "Time taken by a planet to orbit the sun" .)