The most common and most likely answer to this crossword clue is the 5 letter word GULES
I've seen this crossword clue in The Times
G
U
L
E
S
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 = gules.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 GULES mean?
GULES noun- The throat; the gullet.
- Red, e.g. on a coat of arms, typically represented in engraving by vertical parallel lines.
- In blazon, of the colour red.