Cut up tough fish (8)
I believe the answer is:
pilchard
'fish' is the definition.
(pilchard is a kind of fish)
'cut up tough' is the wordplay.
'cut' becomes 'clip' (clip is a kind of cut**).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'tough' becomes 'hard' ('hard' is associated in meaning with 'tough').
'clip' in reverse letter order is 'pilc'.
'pilc'+'hard'='PILCHARD'
(Other definitions for pilchard that I've seen before include "type of fish" , "Fish of herring family" , "Item of seafood" , "'Small fatty fish, usually canned (8)'" , "Small sea fish related to the herring" .)