The most common and most likely answer to this crossword clue is the 9 letter word COMMUNIST
I've seen this crossword clue in The Daily Telegraph
C
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More clues leading to the result COMMUNIST
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 = communist.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 COMMUNIST mean?
COMMUNIST noun- An advocate of a society based on the common ownership of property; a proponent of communism.
- Any revolutionary or subversive radical.
- Of, relating to, supporting, or advocating communism.