Low-key coronation may be so rude (13)

I believe the answer is:

unceremonious

I believe this is a double definition.

'low-key coronation may be so' is the first definition.
(a coronation might lack a ceremony)

'rude' is the second definition.
(thesaurus)

(Another definition for unceremonious that I've seen is " Informal".)

I've seen this clue in The Times.

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