The most common and most likely answer to this crossword clue is the 4 letter word HARK
I've seen this crossword clue in The Daily Telegraph
H
A
R
K
More clues leading to the result HARK
Buddy explains!

The straight part
We know that "12" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 12 = hark.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 HARK mean?
HARK noun- (Scots) A whisper
- To listen attentively; often used in the imperative.