Outbreak (8)
I believe the answer is:
eruption
'outbreak' is the definition.
(I know that outbreak can be written as eruption)
This is all the clue.
(Other definitions for eruption that I've seen before include "Volcanic outpouring" , "which blighted Pompeii?" , "Violent outbreak as by volcano" , "A violent breaking out, as of volcano" , "Spot or rash appearing suddenly on the skin" .)