Fish, most unappetising part (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna fish is a kind of fish)
'most unappetising part' is the wordplay.
'part' indicates the answer is hidden within the clue.
'TUNA' can be found hidden inside 'most unappetising'.
(Other definitions for tuna that I've seen before include "Large sea-fish" , "Fish, often tinned" , "Marine food fish" , "(Meat of) large fish" , "Common food fish" .)