Cross monkeys treated with gas, using these appliances? (6,5)

I believe the answer is:

oxygen masks

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'these appliances?' is the definition.
The definition suggests an adverb but the answer is not.

'cross monkeys treated with gas' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'treated with' indicates anagramming the letters.
'x'+'monkeys'='xmonkeys'
'xmonkeys' anagrammed with 'gas' can make 'OXYGEN MASKS'.

'using' acts as a link.

Can you help me to learn more?

(Another definition for oxygen masks that I've seen is " Special facial covers".)

I've seen this clue in The Daily Mail.

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