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

The straight part
We know that "Weak" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case weak = enervated.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 ENERVATED mean?
ENERVATED verb- To reduce strength or energy; debilitate.
- To weaken morally or mentally.
- To partially or completely remove a nerve.
- Weakened, debilitated or deprived of strength or vitality.