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?" .)