The most common and most likely answer to this crossword clue is the 9 letter word LIGHTENED
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!

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 LIGHTENED mean?
LIGHTENED verb- To make brighter or clearer; to illuminate.
- To become brighter or clearer; to brighten.
- To burst forth or dart, as lightning; to shine with, or like, lightning; to flash.
- To emit or disclose in, or as if in, lightning; to flash out, like lightning.
- To illuminate with knowledge; to enlighten.
- To alleviate; to reduce the burden of.
- To make light or lighter in weight.
- To make less serious or more cheerful.
- To become light or lighter in weight.
- To become less serious or more cheerful.
- To descend; to light.