Spread by dancers falls when they wear masks (5,5,4)

I believe the answer is:

fancy dress ball

I'm a little stuck... Click here to teach me more about this clue!

'they wear masks' is the definition.
The definition suggests an adverb but the answer is not.

'spread by dancers falls' is the wordplay.
'spread' indicates an anagram.
'by'+'dancers'+'falls'='bydancersfalls'
'bydancersfalls' with letters rearranged gives 'FANCY-DRESS BALL'.

'when' is the link.

Can you help me to learn more?

(Another definition for fancy dress ball that I've seen is " social occasion".)

I've seen this clue in The Telegraph.

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