Reveal holiday rash (8)
I believe the answer is:
outbreak
'rash' is the definition.
(outbreak can mean a rash or flare-up)
'reveal holiday' is the wordplay.
'reveal' becomes 'out' (outing is a kind of revealing).
'holiday' becomes 'break' (break can mean a holiday or time off).
'out'+'break'='OUTBREAK'
(Other definitions for outbreak that I've seen before include "Sudden occurrence (of war, disease, etc.)" , "Eruption of e.g. disease" , "Sudden spontaneous occurrence" , "rash" , "Sudden start" .)