The most common and most likely answer to this crossword clue is the 12 letter word DISAPPEARING
I've seen this crossword clue in The Daily Telegraph
D
I
S
A
P
P
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A
R
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More clues leading to the result DISAPPEARING
Buddy explains!

The straight part
We know that "Fading" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fading = disappearing.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DISAPPEARING mean?
DISAPPEARING verb- To vanish.
- To make vanish; especially, to abduct and murder surreptitiously for political reasons.
- To go away; to become lost.
- Disappearance