The most common and most likely answer to this crossword clue is the 8 letter word BLEACHER
I've seen this crossword clue in Financial Times
B
L
E
A
C
H
E
R
Buddy explains!
The straight part
We know that "whitening mechanism" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whitening mechanism = bleacher.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 BLEACHER mean?
BLEACHER noun- One who, or that which, bleaches.
- (usually in the plural) A stand of tiered planks used as seating for spectators, often outdoors.
- (usually in the plural, by extension) The tiered seating provided for the audience at a television filming.