Top banana? (4,2,3,5)

I think it could be either:

best of the bunch

pick of the bunch

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

'banana?' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'top' is the wordplay.
I cannot really see how this works, but
'top' could be 'best' (similar in meaning) and 'best' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Guardian and The Telegraph.

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