Haddock's last fish in bottom of boat (4)
I believe the answer is:
keel
'bottom of boat' is the definition.
(I've seen this before)
'haddock's last fish' is the wordplay.
'last' says to take the final letters.
'fish' becomes 'eel' (eel is a kind of fish).
The last letter of 'haddock' is 'k'.
'k'+'eel'='KEEL'
'in' is the link.
(Other definitions for keel that I've seen before include "Strut along base of ship" , "Bottom of boat" , "Earth (Latin)" , "Structural member of a ship" , "Low part of ship" .)