Leftists less than a hundred strong (4)
I believe the answer is:
reds
'leftists' is the definition.
(red is a kind of leftist)
'less than a hundred strong' is the wordplay.
'less' says the answer is hidden in the clue (the clue less some of the letters in it).
'REDS' is hidden in the letters of 'than hundred strong'.
(Other definitions for reds that I've seen before include "Colourful communists" , "Left-wingers" , "Scarlet communists" , "These are not right" , "Tidies up" .)