The most common and most likely answer to this crossword clue is the 9 letter word NIGHTLONG
I've seen this crossword clue in The Times
N
I
G
H
T
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O
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G
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 NIGHTLONG mean?
NIGHTLONG adjective- Lasting a night (i.e. the duration of one night); lasting all night.
- Through the night.