Old royal fighting legendary boxer? Then there'll be fireworks! (6)

I believe the answer is:

divali

'then there'll be fireworks' is the definition.
(Hindu festival of lights)

'old royal fighting legendary boxer?' is the wordplay.
'old royal' becomes 'Di' (Princess Diana).
'fighting' becomes 'V' (versus).
'legendary boxer?' becomes 'Ali' (Muhammad Ali).
'di'+'v'+'ali'='DIVALI'

(Another definition for divali that I've seen is " Hindu festival; via lid (anag.)".)

I've seen this clue in The Independent.

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