The most common and most likely answer to this crossword clue is the 9 letter word BILLBOARD
I've seen this crossword clue in The Guardian
B
I
L
L
B
O
A
R
D
Buddy explains!
The straight part
We know that "Advertising" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case advertising = billboard.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 BILLBOARD mean?
BILLBOARD noun- A very large outdoor sign, generally used for advertising.
- A flat surface, such as a panel or fence, on which bills are posted; a bulletin board.
- A piece of thick plank, armed with iron plates, and fixed on the bow or fore-channels of a vessel, for the bill or fluke of the anchor to rest on.
- A sprite that always faces the screen, no matter which direction it is looked at from.