Russian leader in square originally leading revolutionary (5)
I believe the answer is:
lenin
'russian leader' is the definition.
(Vladimir Lenin)
'square originally leading revolutionary' is the wordplay.
'square' becomes 'nine' (nine is a square number).
'originally' indicates taking the first letters.
'revolutionary' is a reversal indicator (letters revolve around).
The first letter of 'leading' is 'l'.
'nine'+'l'='ninel'
'ninel' in reverse letter order is 'LENIN'.
'in' acts as a link.
(Other definitions for lenin that I've seen before include "Head of the Communist state after the fall of the Tsar" , "Revolutionary leader" , "First Soviet leader" , "Communist leader" , "V. I. -, Soviet politician and revolutionary" .)