Description of ducks eating toast? (7,2)

I believe the answer is:

bottoms up

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'toast' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for bottoms up that I've seen before include "Toast" , "Down the hatch!" , "Your good health!" , "Cheers!" , "down it goes!" .)

I've seen this clue in the Sydney Morning Herald.

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