The most common and most likely answer to this crossword clue is the 12 letter word SLEEPWALKING
I've seen this crossword clue in The Daily Telegraph
S
L
E
E
P
W
A
L
K
I
N
G
Buddy explains!
The straight part
We know that "Stretching one’s legs out?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stretching one’s legs out? = sleepwalking.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 SLEEPWALKING mean?
SLEEPWALKING verb- To walk and/or perform other actions while sleeping; to somnambulate.
- The act of walking while not conscious or aware of it, during one's sleep.