Silent when moving? Join commandos, perhaps (6)
I believe the answer is:
enlist
'join commandos perhaps' is the definition.
(as in enlisting in the army)
'silent when moving?' is the wordplay.
'when moving?' indicates anagramming the letters (I've seen this in other clues).
'silent' anagrammed gives 'ENLIST'.
(Other definitions for enlist that I've seen before include "Obtain help of" , "Join the military" , "Draft" , "augment army" , "Join the armed forces" .)