The most common and most likely answer to this crossword clue is the 9 letter word BLOODIEST
I've seen this crossword clue in The Times
B
L
O
O
D
I
E
S
T
Buddy explains!

The straight part
We know that "Reddest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reddest = bloodiest.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 BLOODIEST mean?
BLOODIEST adjective- Covered in blood.
- Characterised by bloodshed.
- Used as an intensifier.
- Badly behaved; unpleasant; beastly.