Passing trend (3)
I believe the answer is:
fad
'passing trend' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fad that I've seen before include "A passing fashion or craze" , "A craze or current vogue" , "'Craze, all the rage (3)'" , "Short-lived craze" , "transient success" .)