The most common and most likely answer to this crossword clue is the 6 letter word SICKLY
I've seen this crossword clue in The Guardian
S
I
C
K
L
Y
More clues leading to the result SICKLY
Buddy explains!

The straight part
We know that "Feeble" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case feeble = sickly.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 SICKLY mean?
SICKLY verb- To make (something) sickly.
- To become sickly.
- Frequently ill or in poor health.
- Not in good health; (somewhat) sick.
- (of a plant) Characterized by poor or unhealthy growth.
- Appearing ill, infirm or unhealthy; giving the appearance of illness.
- Shedding a relatively small amount of light; (of light) not very bright.
- Lacking intensity or vigour.
- Associated with poor moral or mental well-being.
- Tending to produce nausea.
- Overly sweet.
- Marked by the occurrence of illness or disease (of a period of time).
- Tending to produce disease or poor health.
- In a sick manner; in a way that reflects or causes sickness.