The most common and most likely answer to this crossword clue is the 11 letter word INEFFECTIVE
I've seen this crossword clue in The Guardian
I
N
E
F
F
E
C
T
I
V
E
More clues leading to the result INEFFECTIVE
Buddy explains!

The straight part
We know that "useless/useless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case useless/useless = ineffective.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 INEFFECTIVE mean?
INEFFECTIVE adjective- Not having the desired effect; ineffectual
- Lacking in ability; incompetent or inadequate