Certain type of pyramid? Sensible not having sex on top (8,5)

I believe the answer is:

platonic solid

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'sensible not having sex on top' is the definition.
I can't tell whether this definition defines the answer.

'certain type of pyramid?' is the wordplay.
'certain type of' becomes 'platonic' (I can't explain this - if you can you should give a lot more credence to this answer).
'pyramid?' becomes 'solid' (pyramid is a kind of solid).
'platonic'+'solid'='PLATONIC SOLID'

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