The implication is one would let in old Scottish lord (7)
I believe the answer is:
macduff
I'm afraid I can't even guess how this answer relates to the clue.
(Other definitions for macduff that I've seen before include "His family killed by Macbeth" , "Slayer of 13" , "Shakespearean character" , "Thane of Fife" , "One told to lay on" .)