The most common and most likely answer to this crossword clue is the 11 letter word LEFT-WINGER
I've seen this crossword clue in The Guardian
L
E
F
T
-
W
I
N
G
E
R
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 = left-winger.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 LEFT-WINGER mean?
LEFT-WINGER noun- A person who espouses left-wing political views.
- A member of the left wing of a party or group.
- A person who plays the position of left wing.