Bottle seal of the royal headwear? (5,3)

I believe the answer is:

crown cap

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'bottle seal of' is the definition.
The definition suggests a singular noun which matches the answer.

'royal headwear?' is the wordplay.
'royal' becomes 'crown' (I can't justify this - if you can you should believe this answer much more).
'headwear?' becomes 'cap' (cap is a type of hat).
'crown'+'cap'='CROWN-CAP'

'the' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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