Shoe and its buckles, set back on one's heels (10)

I believe the answer is:

astonished

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'set back on one's heels' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'shoe and its buckles' is the wordplay.
'buckles' is an anagram indicator.
'shoe'+'and'+'its'='shoeandits'
'shoeandits' with letters rearranged gives 'ASTONISHED'.

Can you help me to learn more?

(Other definitions for astonished that I've seen before include "dumbfounded" , "given a bombshell?" , "Staggered" , "Taken by surprise" , "Gobsmacked" .)

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

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