Couples fired up by Christmas Day dalliance (5)

I believe the answer is:

dyads

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'couples' is the definition.
('dyad' can be a synonym of 'couple')

'fired up by christmas day dalliance' is the wordplay.
'DYADS' is hidden back-to-front within the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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