The most common and most likely answer to this crossword clue is the 8 letter word EXTENSOR
I've seen this crossword clue in The Guardian
E
X
T
E
N
S
O
R
Buddy explains!
The straight part
We know that "one stretching a leg?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one stretching a leg? = extensor.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 EXTENSOR mean?
EXTENSOR noun- A muscle whose contraction extends or straightens a limb or body part.