It means a mass shouting (5,5)

I believe the answer is:

crowd scene

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'it' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'a mass shouting' is the wordplay.
'a mass' becomes 'crowd' (I've seen this in another clue).
'shouting' becomes 'scene' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'crowd'+'scene'='CROWD SCENE'

'means' acts as a link.

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I've seen this clue in the Evening Standard.

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