The most common and most likely answer to this crossword clue is the 9 letter word BLOODSHOT
I've seen this crossword clue in Independent
B
L
O
O
D
S
H
O
T
More clues leading to the result BLOODSHOT
Buddy explains!

The straight part
We know that "red" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case red = bloodshot.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 BLOODSHOT mean?
BLOODSHOT adjective- (of the eyes) reddened and inflamed because of congested blood vessels