The most common and most likely answer to this crossword clue is the 11 letter word CONVENIENCE
I've seen this crossword clue in The Times
C
O
N
V
E
N
I
E
N
C
E
Buddy explains!

The straight part
We know that "useful device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case useful device = convenience.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 CONVENIENCE mean?
CONVENIENCE noun- The quality of being convenient.
- Any object that makes life more convenient; a helpful item.
- A convenient time.
- Clipping of public convenience: a public lavatory.
- To make convenient