Side number (6)

I believe the answer is:

eleven

This clue is a double definition.

'side' is the first definition.
(I've seen this before)

'number?' is the second definition.
(eleven is a kind of number)

(Other definitions for eleven that I've seen before include "Number killed in the Peterloo massacre" , "Smallest positive three-syllable integer" , "Number, on a team perhaps" , "football team, for example" , "Football or cricket team?" .)

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

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