Silent way to join up (6)
I believe the answer is:
enlist
'join up' is the definition.
(I've seen this in another clue)
'silent way' is the wordplay.
'way' is an anagram indicator (another way of arranging the letters).
'silent' is an anagram of 'ENLIST'.
'to' acts as a link.
(Other definitions for enlist that I've seen before include "Enrol - recruit" , "Join the army" , "Sign on -- 13 (anag)" , "Take the Queen's shilling" , "Join the armed services" .)