Marble game popular with a number of spectators (4,5)

I believe the answer is:

ruin agate

'marble' is the definition.
(I've seen this before)

'game popular with a number of spectators' is the wordplay.
'game' becomes 'ru' (rugby union).
'popular' becomes 'in'.
'with' says to put letters next to each other.
'number of spectators' becomes 'gate' (number of people attending a sports event).
'ru'+'in'+'a'+'gate'='RUIN AGATE'

(Another definition for ruin agate that I've seen is " Marble".)

I've seen this clue in The Times.

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