Note your rival — somehow radically different (13)
I believe the answer is:
revolutionary
'radically different' is the definition.
'note your rival somehow' is the wordplay.
'somehow' is an anagram indicator.
'note'+'your'+'rival'='noteyourrival'
'noteyourrival' is an anagram of 'REVOLUTIONARY'.
(Other definitions for revolutionary that I've seen before include "French calendar?" , "Innovative" , "agitator" , "New" , "Causing dramatic change" .)