Fish pasty containing last of haddock (4)
I believe the answer is:
pike
'fish' is the definition.
(pike is a kind of fish)
'pasty containing last of haddock' is the wordplay.
'pasty' becomes 'pie' (I've seen this before).
'containing' indicates putting letters inside.
'last of' indicates one should take the final letters.
The final letter of 'haddock' is 'k'.
'pie' enclosing 'k' is 'PIKE'.
(Other definitions for pike that I've seen before include "dive in this position" , "Fresh-water fish or old spear" , "Fish - long spear-like weapon" , "Old spear or voracious lake fish" , "Weapon on long shaft; Northern hill" .)