Silent characters sign up (6)
I believe the answer is:
enlist
'sign up' is the definition.
(enlisting is a kind of signing up)
'silent characters' is the wordplay.
'characters' is an anagram indicator.
'silent' anagrammed gives 'ENLIST'.
(Other definitions for enlist that I've seen before include "Engage" , "Join up - tinsel (anag)" , "Join the military" , "Listen (anag.)" , "Sign on -- 13 (anag)" .)