The most common and most likely answer to this crossword clue is the 10 letter word SCRAWNIEST
S
C
R
A
W
N
I
E
S
T
More clues leading to the result SCRAWNIEST
Buddy explains!

The straight part
We know that "Leanest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leanest = scrawniest.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 SCRAWNIEST mean?
SCRAWNIEST adjective- Thin, malnourished and weak.