The most common and most likely answer to this crossword clue is the 6 letter word INFIRM
I've seen this crossword clue in The Sunday Telegraph
I
N
F
I
R
M
More clues leading to the result INFIRM
- Part of the company’s stable? Hardly
- One northern company is in poor health
- Hot male filled with endless passion being 27
- Popular company’s not in good shape
- Tell Philistine to banish love that’s weak
- Feeble, but employed by business
- Feeble in company
- Failing business goes under at home
- Advise having institute for old and frail
- Trendy company needing support?
Buddy explains!

The straight part
We know that "not well" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not well = infirm.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 INFIRM mean?
INFIRM verb- To contradict, to provide proof that something is not.
- Weak or ill, not in good health.
- Irresolute; weak of mind or will.
- Frail; unstable; insecure.