Happening, so to speak (3,3,4)
I believe the answer is:
all the rage
(Other definitions for all the rage that I've seen before include "The in thing" , "Very much in fashion" , "Very popular (informal)" , "ultra-fashionable" , "in vogue?" .)
I believe the answer is:
all the rage
(Other definitions for all the rage that I've seen before include "The in thing" , "Very much in fashion" , "Very popular (informal)" , "ultra-fashionable" , "in vogue?" .)