Assume goose finally to be cooked (4)
I believe the answer is:
done
'be cooked' is the definition.
('done' can be similar in meaning to 'cooked')
'assume goose finally' is the wordplay.
'assume' becomes 'don' (I've seen this before).
'finally' suggests the final letters.
The last letter of 'goose' is 'e'.
'don'+'e'='DONE'
'to' acts as a link.
(Other definitions for done that I've seen before include "Thoroughly cooked; accomplished" , "Performed" , "Concluded (and dusted?)" , "agreed" , "(Of food) well cooked" .)