Crossword Genius

Filling for huntsman's canine is loose (4)

Ross

I believe the answer is:

undo

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

I cannot understand how the rest of the clue works.

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(Other definitions for undo that I've seen before include "Cancel, annul; open" , "Loosen (knot)" , "(4)" , "Unfasten - destroy" , "Unfasten or annul" .)

I've seen this clue in The Independent.
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