Number of regressive Leninists (4)
I believe the answer is:
nine
'number' is the definition.
(nine is a kind of number)
'of regressive leninists' is the wordplay.
'regressive' shows that the letters should be reversed in order (to regress is to go backwards).
'of' indicates the answer is hidden within the clue.
'enin' can be found hidden inside 'leninists'.
'enin' reversed gives 'NINE'.
(Other definitions for nine that I've seen before include "Number for a club" , "dancing ladies?" , "Cat's lives? - the next clue?" , "Days Lady Jane 5 ac reigned" , "Square number" .)