The most common and most likely answer to this crossword clue is the 8 letter word ERUPTION
I've seen this crossword clue in The Guardian
E
R
U
P
T
I
O
N
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 = eruption.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 ERUPTION mean?
ERUPTION noun- A violent ejection, such as the spurting out of lava from a volcano.
- A sudden release of pressure or tension.
- An infection of the skin resulting in a rash or blemishing.