Eruption? Not in holiday period (8)
I believe the answer is:
outbreak
'eruption?' is the definition.
(I know that eruption can be written as outbreak)
'not in holiday period' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'holiday period' becomes 'break' (eg break from work).
'out'+'break'='OUTBREAK'
(Other definitions for outbreak that I've seen before include "Sudden start" , "Sudden spontaneous occurrence" , "Eruption of e.g. disease" , "Sudden onset (of war, disease, etc)" , "Violent occurrence" .)