Fish pasty filled with tail of haddock (4)
I believe the answer is:
pike
'fish' is the definition.
(pike is a kind of fish)
'pasty filled with tail of haddock' is the wordplay.
'pasty' becomes 'pie' (I've seen this before).
'filled with' indicates putting letters inside.
'tail of' 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 "Imitating earlier style" , "Long European freshwater predator" , "Predator with strong teeth" , "dive in this position" , "Fish - medieval weapon" .)