Join the military for silent activity (6)
I believe the answer is:
enlist
'join the military' is the definition.
(I've seen this in another clue)
'silent activity' is the wordplay.
'activity' is an anagram indicator (letters become active).
'silent' with letters rearranged gives 'ENLIST'.
'for' is the link.
(Other definitions for enlist that I've seen before include "Sign on -- 13 (anag)" , "Join the armed services" , "Enrol in armed forces" , "Take the Queen's shilling" , "augment army" .)