The most common and most likely answer to this crossword clue is the 8 letter word BYE-BYES
I've seen this crossword clue in The Guardian
B
Y
E
-
B
Y
E
S
Buddy explains!

The straight part
We know that "Lights out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lights out = bye-byes.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 BYE-BYES mean?
BYE-BYES noun- A goodbye.
- Bedtime for a toddler, going to sleep, going to bed.