Not the sole order dogs learn to obey (4)

I believe the answer is:

heel

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

I cannot understand how the rest of the clue works.

(Other definitions for heel that I've seen before include "Spur" , "afoot?" , "bank" , "Untrustworthy person" , "See 1 down" .)

I've seen this clue in The Irish Independent and The Globe and Mail.

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