The most common and most likely answer to this crossword clue is the 9 letter word CANVASSER
I've seen this crossword clue in The Daily Telegraph
C
A
N
V
A
S
S
E
R
Buddy explains!
The straight part
We know that "Door-to-door operator" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case door-to-door operator = canvasser.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.
What does CANVASSER mean?
CANVASSER noun- Someone who goes through a region soliciting votes in an election, or conducting a public opinion poll