Crossword Genius

Sort of cap for a bonehead? (5)

Ross

I believe the answer is:

skull

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

This might be a double definition.

'sort of cap' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'a bonehead?' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for skull that I've seen before include "Bony frame of head ..." , "Bony part of head" , "Part of skeleton" , "Head structure" , "Brain casing" .)

I've seen this clue in the Evening Standard.
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