The most common and most likely answer to this crossword clue is the 7 letter word ROSEOLA
I've seen this crossword clue in The Guardian
R
O
S
E
O
L
A
More clues leading to the result ROSEOLA
Buddy explains!

The straight part
We know that "Rash" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rash = roseola.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 ROSEOLA mean?
ROSEOLA noun- A rosy rash occurring in measles, typhoid fever, syphilis and some other diseases.
- The common name for roseola infantum.
- German measles.