Stuff that’s over buns? (6)

I believe the answer is:

glutei

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'buns?' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.

'stuff that's over' is the wordplay.
'stuff' becomes 'glut' (synonyms).
'that's' becomes 'ie'.
'over' shows that the letters should be reversed in order.
'ie' back-to-front is 'ei'.
'glut'+'ei'='GLUTEI'

Can you help me to learn more?

(Other definitions for glutei that I've seen before include "behind?" , "muscles" .)

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

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