The Clerk of the Parliaments is the most senior official in the House of Lords. The position is like that of a Chief Executive. He is responsible for employing staff in the House of Lords, advising the Lord Speaker and the Leader of the House and on the accuracy of the text of Acts. He is also the Registrar of the Court when the House of Lords performs its judicial role.
Further information on this subject can be found from the following links.
The Clerk of the Parliaments: role and functions
Interview with the Clerk of the Parliaments