Revolutionary line has upset number (5)
I believe the answer is:
lenin
'revolutionary' is the definition.
(I've seen this before)
'line has upset number' is the wordplay.
'has upset' indicates anagramming the letters.
'number' becomes 'n'.
'line' is an anagram of 'leni'.
'leni'+'n'='LENIN'
(Other definitions for lenin that I've seen before include "First Soviet leader" , "Soviet Union's first premier" , "Linen for Communist" , "Early Soviet leader" , "Revolutionary leader" .)