Prosperous period (4)
I believe the answer is:
boom
'prosperous period' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boom that I've seen before include "time of 22 [PROSPERITY]" , "Period of prosperity needed" , "Deep sound; spar" , "Deep and resonant sound" , "Resound - spar - economic growth" .)