The last piece of fish, a cut of halibut (5)
I believe the answer is:
final
'the last' is the definition.
(I've seen this in another clue)
'piece of fish a cut of halibut' is the wordplay.
'piece of fish' becomes 'fin' (fish have fins).
'a cut of halibut' becomes 'al' (I can't explain this - if you can you should give a lot more credence to this answer).
'fin'+'al'='FINAL'
Can you help me to learn more?
(Other definitions for final that I've seen before include "Last in a series of games or contests" , "binding" , "Last match in a competition" , "Concluding" , "big match" .)