At least fourteen in cathedral's evensong? (6)

I believe the answer is:

sevens

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'at least fourteen' is the definition.
I can't tell whether this definition defines the answer.

'in cathedral's evensong?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SEVENS' can be found hidden inside 'cathedrals evensong'.

Can you help me to learn more?

(Other definitions for sevens that I've seen before include "cards!" , "Card game; form of rugby" , "Form of Rugby Union" , "77" , "rugby competition" .)

I've seen this clue in The Guardian.

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