The most common and most likely answer to this crossword clue is the 17 letter word GOOD-FOR-NOTHINGS
I've seen this crossword clue in The Times
G
O
O
D
-
F
O
R
-
N
O
T
H
I
N
G
S
More clues leading to the result GOOD-FOR-NOTHINGS
Buddy explains!
The straight part
We know that "they're useless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they're useless = good-for-nothings.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 GOOD-FOR-NOTHINGS mean?
GOOD-FOR-NOTHINGS noun- A person of little worth or usefulness.