The most common and most likely answer to this crossword clue is the 6 letter word FUTILE
I've seen this crossword clue in The Guardian
F
U
T
I
L
E
Buddy explains!

The straight part
We know that "useless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case useless = futile.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 FUTILE mean?
FUTILE adjective- Incapable of producing results; doomed not to be successful; not worth attempting.