Filling for huntsman's canine is loose (4)
I believe the answer is:
undo
'is loose' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for undo that I've seen before include "Cancel, annul; open" , "Loosen (knot)" , "(4)" , "Unfasten - destroy" , "Unfasten or annul" .)