The most common and most likely answer to this crossword clue is the 7 plus 5 letter word BURGLAR ALARM
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More clues leading to the result BURGLAR ALARM
Buddy explains!
The straight part
We know that "It goes off when it’s on if one’s in when you’re out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it goes off when it’s on if one’s in when you’re out = burglar alarm.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.