The most common and most likely answer to this crossword clue is the 8 letter word PETECHIA
I've seen this crossword clue in Independent
P
E
T
E
C
H
I
A
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The straight part
We know that "Red spot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case red spot = petechia.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 PETECHIA mean?
PETECHIA noun- A small spot, especially on an organ, caused by bleeding underneath the skin.