Thousands and thousands initially inside place where 1/21, 9, 10, 12/20 and 24/26 might be seen (10)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

[ GRANDS(plural of a £1000 or $1000) + AND ] containing ( … inside ) 1st letter of ( … initially ) “ thousands ”. Defn: …/stand from where one might watch the aforementioned sportswomen and sportsmen playing their matches.