Score fantastic ton going in — or a duck (6)
I believe the answer is:
scoter
'a duck' is the definition.
(scoter is a kind of duck)
'score fantastic ton going in' is the wordplay.
'fantastic' is an anagram indicator.
'ton' becomes 't'.
'going in' is an insertion indicator.
'score' is an anagram of 'scoer'.
'scoer' placed around 't' is 'SCOTER'.
'or' is the link.
(Other definitions for scoter that I've seen before include "Northern diving duck" , "Duck - escort (anag)" , "Species of diving duck" , "Sea duck" , "Diving duck with black plumage" .)