Clue: Person admitting deficiency, right person to knock on door (5,3)
Answer: BLACK ROD
Explanation: Definition: person to knock on door
**BOD** ("person") *admitting* **LACK** ("deficiency") + **R**(Ight) 
*The usher of the House of Lords who summons the House of Commons at the opening session of the UK Parlaiment is called Black Rod. He knocks loudly on the door of the House of Commons as part of the ceremony.