Rebel, Republican in transition (13)
I believe the answer is:
revolutionary
'rebel' is the definition.
(thesaurus)
'republican in transition' is the wordplay.
'republican' becomes 'r' (abbreviation).
'in transition' becomes 'evolutionary'.
'r'+'evolutionary'='REVOLUTIONARY'
(Other definitions for revolutionary that I've seen before include "New" , "Insurgent - involving dramatic change" , "agitator" , "Innovative" , "French calendar?" .)