The most common and most likely answer to this crossword clue is the 5 plus 3 letter word BLACK ROD
I've seen this crossword clue in The Guardian
B
L
A
C
K
R
O
D
Buddy explains!
The straight part
We know that "person to knock on door" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case person to knock on door = black rod.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.