Number of Leninists turning up (4)
I believe the answer is:
nine
'number' is the definition.
(nine is a kind of number)
'of leninists turning up' is the wordplay.
'of' says the answer is hidden in the clue.
'turning up' says the letters should be written backwards.
'enin' can be found hidden inside 'leninists'.
'enin' backwards is 'NINE'.
(Other definitions for nine that I've seen before include "Three squared" , "Number of Muses and days in a Novena" , "Number of lepers who failed to return" , "Number of players in a baseball fielding team" , "One more than fourteen across" .)