Opted for removing head of horrible weapon (7)
I believe the answer is:
torpedo
'weapon' is the definition.
(weapon used at sea)
'opted for removing head of horrible' is the wordplay.
'removing head' means to remove the first letter.
'of horrible' indicates an anagram (I've seen 'horrible' mean this).
'for' with its initial letter taken off is 'or'.
'opted'+'or'='optedor'
'optedor' is an anagram of 'TORPEDO'.
(Other definitions for torpedo that I've seen before include "Sub's missile" , "[SOMETHING FISHY]" , "Naval missile" , "Missile form a submarine" , "Trooped (anag.)" .)