The most common and most likely answer to this crossword clue is the 7 letter word CARMINE
I've seen this crossword clue in The Times
C
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M
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The straight part
We know that "Vivid red" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vivid red = carmine.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 CARMINE mean?
CARMINE noun- A purplish-red pigment, made from dye obtained from the cochineal beetle; carminic acid or any of its derivatives.
- A purplish-red colour, resembling that pigment.
- Of the purplish red colour shade carmine.