The most common and most likely answer to this crossword clue is the 13 letter word MERCHANDISING
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Advertising material" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case advertising material = merchandising.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 MERCHANDISING mean?
MERCHANDISING verb- To engage in trade; to carry on commerce.
- To engage in in-store promotion of the sale of goods, as by display and arrangement of goods.
- To engage in the trade of.
- To engage in in-store promotion of the sale of.
- To promote as if for sale.
- Trade in merchandise