Dreadful private school grad with debts stuck outside No 10 (9)

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

OB (Old Boy [graduate of a private school]- I’ve always assumed Old Boy could apply to someone who went to any school] + IOUS (I owe yous – debts]) containing (stuck outside) ( NO [number] + X [Roman numeral for 10])