Public holiday causes epidemic (8)
I believe the answer is:
outbreak
'epidemic' is the definition.
(thesaurus)
'public holiday' is the wordplay.
'public' becomes 'out' (I've seen this before).
'holiday' becomes 'break' (I've seen this before).
'out'+'break'='OUTBREAK'
'causes' is the link.
(Other definitions for outbreak that I've seen before include "Violent occurrence" , "rash" , "Sudden start" , "Sudden occurrence (of war, disease, etc.)" , "disturbance" .)