Spa offers these in France: Calais possibly (7)

I believe the answer is:

facials

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'spa offers' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.

'these in france calais possibly' is the wordplay.
I cannot quite understand how this works, but
'france' could be 'f' and 'f' is found in the answer.
an anagram of 'calais' is 'acials' which is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

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I've seen this clue in The Sun.

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