The most common and most likely answer to this crossword clue is the 9 letter word TIREDNESS
I've seen this crossword clue in The Sunday Telegraph
T
I
R
E
D
N
E
S
S
More clues leading to the result TIREDNESS
Buddy explains!

The straight part
We know that "Lack of energy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lack of energy = tiredness.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 TIREDNESS mean?
TIREDNESS noun- The state of being tired.