Public holiday's sudden start (8)
I believe the answer is:
outbreak
'sudden start' is the definition.
(I've seen this before)
'public holiday's' is the wordplay.
'public' becomes 'out' (out can mean released to the public).
'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.)" , "Sudden development of disease etc" , "rash" , "Sudden start" , "Onset -- epidemic" .)