A royal booze-up (5)

I believe the answer is:

regal

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'a royal' is the definition.
(I know that royal can be written as regal)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for regal that I've seen before include "Kingly or queenly" , "Like a queen?" , "Fit for a monarch" , "fit for the Queen" , "befitting a king" .)

I've seen this clue in the Evening Standard.

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