Fish, duck and butter twists (6)
I believe the answer is:
marlin
'fish' is the definition.
(type of fish)
'duck and butter twists' is the wordplay.
'duck' becomes 'nil' (both can mean zero).
'and' says to put letters next to each other.
'butter' becomes 'ram' (a ram is something which 'butts').
'twists' is a reversal indicator (I've seen 'twisted ' mean this).
'nil'+'ram'='nilram'
'nilram' back-to-front is 'MARLIN'.
(Other definitions for marlin that I've seen before include "Sea roamer" , "Large ocean fish" , "Prized game fish" , "type of fish" , "Spearfish" .)