Things that last a very short time (8)
I believe the answer is:
ephemera
'things that last a very short time' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ephemera that I've seen before include "Highly transient things" , "Flyer" , "Something that lasts or is useful for only a short time" , "Mayflies" , "They won't last long" .)