How many make up a Spanish soccer side at any time? (4)

I believe the answer is:

once

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'at any time?' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for once that I've seen before include "A single time" , "Cone (anag.)" , "On a single (or past) occasion" , "not repeatedly" , "Quick examination" .)

I've seen this clue in The Telegraph.

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