Jolly Roger’s head case (5)

I believe the answer is:

skull

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

'head case' is the definition.
'skull' can be an answer for 'head' (I've seen this before). I'm not certain of the 'case' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for skull that I've seen before include "Pirate flag feature" , "Bony part of head" , "Bony skeleton of the head" , "Head structure" , "Bony frame of head ..." .)

I've seen this clue in the Evening Standard.

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