Sudden flood of arrivals ruins new hospital (6)
I believe the answer is:
inrush
'sudden flood of arrivals' is the definition.
'ruins new hospital' is the wordplay.
'new' indicates anagramming the letters.
'hospital' becomes 'h'.
'ruins' with letters rearranged gives 'inrus'.
'inrus'+'h'='INRUSH'
(Other definitions for inrush that I've seen before include "Overwhelming flow" , "Rapid flow from outside" , "Sudden arrival or entry" , "Crowd invasion" , "Sudden influx" .)