The most common and most likely answer to this crossword clue is the 6 letter word MUCOSA
M
U
C
O
S
A
Buddy explains!
The straight part
We know that "It prevents dust from getting in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it prevents dust from getting in = mucosa.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 MUCOSA mean?
MUCOSA noun- Mucous membrane