Aristocracy in top form (5-5)

I believe the answer is:

upper-class

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'aristocracy' is the definition.
(I've seen this in another clue)

'in top form' is the wordplay.
'in top' becomes 'upper' (similar in meaning. I am not sure about the 'in' bit.).
'form' becomes 'class' (synonyms).
'upper'+'class'='UPPER-CLASS'

Can you help me to learn more?

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

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