A bit of a loaf for aristocrats? (3,5,5)

I believe the answer is:

the upper crust

I'm a little stuck... Click here to teach me more about this clue!

'loaf for aristocrats?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'a bit' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the answer.
This explanation may well be incorrect...

'of a' acts as a link.

Can you help me to learn more?

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