A fishy old weapon (4)
I believe the answer is:
pike
This clue could be a double definition.
'a fishy' is the first definition.
(type of fish)
'old weapon' is the second definition.
'pike' can be an answer for 'weapon' (pike is a kind of weapon). I am unsure of the 'old' bit.
Can you help me to learn more?
(Other definitions for pike that I've seen before include "jackknife" , "Former infantry weapon" , "Shafted weapon; diving position" , "See 1 down" , "Fish - old infantry weapon" .)