Fat is best after cake (6,3)

I believe the answer is:

muffin top

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'fat' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'best after cake' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'after' means one lot of letters go next to another.
'cake' becomes 'muffin' (I've seen this before).
'top' after 'muffin' is 'MUFFIN TOP'.

'is' acts as a link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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