The most common and most likely answer to this crossword clue is the 8 letter word HYGIENIC
I've seen this crossword clue in The Daily Telegraph
H
Y
G
I
E
N
I
C
Buddy explains!

The straight part
We know that "clean" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clean = hygienic.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 HYGIENIC mean?
HYGIENIC adjective- Pertaining to hygiene; clean, sanitary.
- (of a macro) Whose expansion is guaranteed not to cause the accidental capture of identifiers.