Top banana? (4,2,3,5)
'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?
But I've got no explanation for this one! Could you help me understand?