Join silent order (6)
I believe the answer is:
enlist
'join' is the definition.
(I know that enlist is a more specific form of the action sign up)
'silent order' is the wordplay.
'order' indicates an anagram.
'silent' with letters rearranged gives 'ENLIST'.
(Other definitions for enlist that I've seen before include "Join the armed services" , "Join up, sign on" , "Join the military" , "Enrol - recruit" , "Take the Queen's shilling" .)