One looking jolly like Roger? (5)

I believe the answer is:

skull

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'one looking jolly like roger?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

This is all the clue.

Can you help me to learn more?

(Other definitions for skull that I've seen before include "Pirate flag feature" , "Bony frame of head ..." , "The head-bone" , "Brain casing" , "Bony skeleton of the head" .)

I've seen this clue in the Evening Standard.

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