Last portion of fish, a tail end (5)
I believe the answer is:
final
'last' is the definition.
(I know that last can be written as final)
'portion of fish a tail end' is the wordplay.
'portion of fish' becomes 'fin' (fish have fins).
'end' suggests the final letters.
The final letter of 'tail' is 'l'.
'fin'+'a'+'l'='FINAL'
(Other definitions for final that I've seen before include "Not to be altered" , "Like one's journey to cemetery" , "The last exam or leg of competition" , "Concluding" , "Coming at the end" .)