Cosmic principle adopted by sovereign a year after uprising (4)
I believe the answer is:
yang
'cosmic principle' is the definition.
(as in yin and yang)
'adopted by sovereign a year after uprising' is the wordplay.
'adopted by' indicates a hidden word.
'after uprising' says the letters should be written backwards.
'gnay' is hidden within 'sovereign a year'.
'gnay' written backwards gives 'YANG'.
(Other definitions for yang that I've seen before include "Yin's counterpart" , "Yin counterpart" , "positive force" , "Positive element in traditional Chinese philosophy" , "Chinese male principle" .)