The most common and most likely answer to this crossword clue is the 8 letter word HELPLESS
I've seen this crossword clue in The Guardian
H
E
L
P
L
E
S
S
Buddy explains!

The straight part
We know that "Unable to manage" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unable to manage = helpless.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 HELPLESS mean?
HELPLESS adjective- Unable to defend oneself.
- Lacking help; powerless.
- Unable to act without help; needing help; feeble.
- Uncontrollable.
- From which there is no possibility of being saved.