The most common and most likely answer to this crossword clue is the 14 letter word UNCONTAMINATED
I've seen this crossword clue in The Times
U
N
C
O
N
T
A
M
I
N
A
T
E
D
More clues leading to the result UNCONTAMINATED
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 = uncontaminated.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 UNCONTAMINATED mean?
UNCONTAMINATED adjective- Not contaminated; unpolluted.