Decapitate spear fish (4)
I believe the answer is:
pike
'fish' is the definition.
(pike is a kind of fish)
'decapitate spear' is the wordplay.
'decapitate' means to remove the first letter (I've seen 'decapitating' mean this).
'spear' becomes 'spike' (I've seen this before).
'spike' with its first letter taken away is 'PIKE'.
(Other definitions for pike that I've seen before include "Old weapon -- Dad's Army character" , "dive in this position" , "may be hooked" , "Spearhead attached to a long pole" , "Fish; weapon; hill" .)