Military headgear, just quietly? A knock out! (5)
I believe the answer is:
shako
'military headgear' is the definition.
(I've seen this in another clue)
'quietly? a knock out' is the wordplay.
'quietly?' becomes 'sh' (both are used to tell someone to be quiet).
'knock out' becomes 'ko'.
'sh'+'a'+'ko'='SHAKO'
'just' is the link.
(Other definitions for shako that I've seen before include "Plumed military cap" , "Old mil. headdress" , "military headdress" , "One covering for military head" , "hat from soldier" .)