The most common and most likely answer to this crossword clue is the 11 plus 4 letter word COMBINATION LOCK
I've seen this crossword clue in The Daily Telegraph
C
O
M
B
I
N
A
T
I
O
N
L
O
C
K
More clues leading to the result COMBINATION LOCK
- Means to open case to nick Colombian criminal
- Search one country to secure safe access?
- Cracking material with a sequence of opening numbers
- Safe device I used after brush with people getting forward
- Security for motorbike and sidecar?
- 5001001000?
- In limbo, cook can’t reset safety device
- Search people following one group avoiding fine security device
Buddy explains!
The straight part
We know that "Safe device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case safe device = combination lock.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.