What is Ad Admittendum Clericum?

Legal Definition
For the admitting of the clerk. A writ in the nature of an execution, commanding the bishop to admit his clerk, upon the success of the latter in a quare impedit.
-- Black's Law Dictionary
Legal Definition
A writ by which a bishop was required to admit and institute the clerk of the party who has been successful in a contest for an office. See 3 Bl. Comm. 250.
-- Ballentine's Law Dictionary