One is against another football team (6)

I believe the answer is:

eleven

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'football team' is the definition.
(I know that football team can be written as eleven)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for eleven that I've seen before include "Number of pipers piping" , "XI as Roman numeral" , "Time for the mid-morning tea break?" , "Number on a soccer team" , "Prime number" .)

I've seen this clue in The Globe and Mail.

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