Fish; medieval weapon (4)
I believe the answer is:
pike
I believe this is a double definition.
'fish' is the first definition.
(pike is a kind of fish)
'medieval weapon' is the second definition.
(I've seen this in another clue)
(Other definitions for pike that I've seen before include "Voracious freshwater fish" , "Fresh-water fish or old spear" , "Weapon on long shaft; Northern hill" , "Fish; weapon; hill" , "Position of one diving" .)