Period of prosperity (4)
I believe the answer is:
boom
'period of prosperity' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boom that I've seen before include "Dull explosion; prosperous period" , "Noise of large explosion" , "Resonant sound < of TV equipment" , "Resound - spar - economic growth" , "It may protect harbour" .)