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