Fish; weapon (4)
I believe the answer is:
pike
This clue is a double definition.
'fish' is the first definition.
(pike is a kind of fish)
'weapon' is the second definition.
(I know that pike is a type of weapon)
(Other definitions for pike that I've seen before include "Weapon on long shaft; Northern hill" , "Shafted weapon; diving position" , "Predatory freshwater fish" , "Fish - long spear-like weapon" , "Fresh-water fish or old spear" .)