Thriving time (4)
I believe the answer is:
boom
'thriving time' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for boom that I've seen before include "Bonanza; roar" , "When business is great, opposite to bust" , "Deep sound; spar" , "Period of prosperity needed" , "Big bang sound" .)