Mostly long revolutionary period (4)
I believe the answer is:
year
'revolutionary period' is the definition.
(the Earth completing a revolution around the Sun)
'mostly long' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'long' becomes 'yearn' (synonyms).
'yearn' with its final letter taken off is 'YEAR'.
(Other definitions for year that I've seen before include "1066, for example" , "365 or 366 days" , "Orbital period" , "One tenth of a decade" , "See 19" .)