The most common and most likely answer to this crossword clue is the 9 letter word CHALKIEST
I've seen this crossword clue in Independent
C
H
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Buddy explains!

The straight part
We know that "as white and dusty as can be" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as white and dusty as can be = chalkiest.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 CHALKIEST mean?
CHALKIEST adjective- Consisting of or containing chalk.
- Resembling chalk in some way.
- Of a tournament: in which the favorites win, or expected to win, most of the games.