Being short-lived, it starts fading (3)
I believe the answer is:
fad
'being' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fad that I've seen before include "Passionate interest" , "Caprice" , "Personal whim" , "A passing fashion or craze" , "Passing vogue or fashion" .)