The most common and most likely answer to this crossword clue is the 12 letter word PLASTERBOARD
I've seen this crossword clue in The Times
P
L
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Buddy explains!
The straight part
We know that "material applied to walls" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case material applied to walls = plasterboard.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 PLASTERBOARD mean?
PLASTERBOARD noun- A construction material consisting of a rigid panel of several layers of fibreboard or paper bonded to a gypsum core.
- To fit or reinforce with plasterboard.