The most common and most likely answer to this crossword clue is the 8 letter word IMPOTENT
I've seen this crossword clue in The Guardian
I
M
P
O
T
E
N
T
More clues leading to the result IMPOTENT
- Old red wine added to spirit that's weak
- Little devil by old shelter, unable to act
- Young troublemaker, ordinary figure close to delinquent lacking strength
- Independent parliamentarian to back hospital department without effect
- Helpless monkey returned to hospital department
- Weak figure entering testament in support of the devil
- Trouble-maker, having taken old red wine, unable to act
- Dispensing with loving gestures, thugs pin me to ground, helpless
- Ineffective for one redcap round shelter
- Prepared? No, I tempt without effect
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 = impotent.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 IMPOTENT mean?
IMPOTENT noun- A man who has erectile dysfunction
- An impotent or powerless person
- Lacking physical strength or vigor; weak
- Lacking in power, as to act effectively; helpless
- Incapable of sexual intercourse, often because of an inability to achieve or sustain an erection
- (of a male) sterile
- Lacking self-restraint