The most common and most likely answer to this crossword clue is the 9 letter word BACKSPACE
I've seen this crossword clue in The Times
B
A
C
K
S
P
A
C
E
Buddy explains!

The straight part
We know that "Key" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case key = backspace.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 BACKSPACE mean?
BACKSPACE noun- The key on a typewriter that moves the head one position backwards.
- A keyboard key used for removing a character behind the cursor, and moving the cursor one position backwards.
- The non-printable text character representing a backspace.
- To remove a character behind a cursor.
- To move a magnetic tape to a previous block.