Fish dish containing haddock tail (4)
I believe the answer is:
pike
'fish' is the definition.
(pike is a kind of fish)
'dish containing haddock tail' is the wordplay.
'dish' becomes 'pie' (I've seen this before).
'containing' indicates putting letters inside.
'tail' suggests the final letters.
The last letter of 'haddock' is 'k'.
'pie' going around 'k' is 'PIKE'.
(Other definitions for pike that I've seen before include "Position of one diving" , "Fresh-water fish or old spear" , "Voracious freshwater fish" , "Predatory freshwater fish" , "dive in this position" .)