Haddock’s last fish in boat's bottom (4)
I believe the answer is:
keel
'boat's bottom' is the definition.
(I've seen this in another clue)
'haddock's last fish' is the wordplay.
'last' indicates one should 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 "Longitudinal structure supporting the frame of a vessel" , "Structure along base of ship" , "Ship's bottom" , "Long part on a boat's bottom" , "Beam on the hull of a vessel" .)