The agency takes 35% of all my commissions before I am paid.
I note you state they take their cut "BEFORE" you are paid. If they issue you a 1099 and the amount they take is "NOT" included on the 1099, then you have nothing to deduct or claim on your SCH C for their cut.
Otherwise, if the total amount paid by all clients is included on the 1099 they send you, then you can deduct their cut from that total. You'll also need to send them a 1099 for the total amount of their cut paid during the tax year.
If they do not send you a 1099 and you only report the amount they pay you "AFTER" they take their cut, then you have nothing to deduct.