The most common and most likely answer to this crossword clue is the 9 letter word INANITION
I
N
A
N
I
T
I
O
N
Buddy explains!

The straight part
We know that "lassitude" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lassitude = inanition.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 INANITION mean?
INANITION noun- The act of removing the contents of something; the state of being empty.
- A state of advanced lack of adequate nutrition, food, or water or a physiological inability to utilize them, with resulting weakness; starvation or cachexia.
- A spiritual emptiness or lack of purpose or will to live, akin to the Existentialist Philosophy state of "nausea".