Blackmail former number one communist after revolution (9)
I believe the answer is:
extortion
'blackmail' is the definition.
(I've seen this in another clue)
'former number one communist after revolution' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'number one' becomes 'no i' (resembles No 1).
'communist' becomes 'trot' (short for Trotskyist).
'after revolution' is a reversal indicator (letters revolve back).
'noi'+'trot'='noitrot'
'noitrot' reversed gives 'tortion'.
'ex'+'tortion'='EXTORTION'
(Other definitions for extortion that I've seen before include "The obtaining of money by force or threat" , "Extraction of money with menaces" , "Blackmail" .)