Silent revolutionary holding small signs up (7)
I believe the answer is:
enlists
'signs up' is the definition.
(enlisting is a kind of signing up)
'silent revolutionary holding small' is the wordplay.
'revolutionary' indicates anagramming the letters.
'holding' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size**).
'silent' anagrammed gives 'enlist'.
'enlist'+'s'='ENLISTS'
(Other definitions for enlists that I've seen before include "Joins up" , "Recruits" , "volunteers" , "Listens (anag.)" , "gets involved" .)