The most common and most likely answer to this crossword clue is the 9 letter word HIBERNATE
H
I
B
E
R
N
A
T
E
More clues leading to the result HIBERNATE
- Breathe in, out, sleep long and deep
- Prepare the brain to conserve energy?
- Breathe in easily and sleep for a while
- Pass winter quietly, ordering herb tea in
- Two thirds of Irish had a meal and sleep
- Hurry round capital city, costing sleep
- Bed down for winter when the brain becomes fuzzy around 4th of November
- Breathe in, when gone to sleep in winter
- Drug in the brain produced sleep over a long period
- Be in earth, out of it?
Buddy explains!

The straight part
We know that "sleep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sleep = hibernate.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 HIBERNATE mean?
HIBERNATE verb- To spend winter time in hibernation.
- To live in seclusion.
- To enter a standby state which conserves power without losing the contents of memory.