Weapon code, protective, partially backed (7)
I believe the answer is:
torpedo
'weapon' is the definition.
(weapon used at sea)
'code protective partially backed' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'backed' says the letters should be written backwards.
'odeprot' is hidden in the letters of 'code protective'.
'odeprot' back-to-front is 'TORPEDO'.
(Other definitions for torpedo that I've seen before include "Sub's missile" , "Missile form a submarine" , "Self-propelled underwater explosive device" , "Self-propelled submarine weapon" , "Missile fired from underwater submarine" .)