The most common and most likely answer to this crossword clue is the 11 letter word ANSWERPHONE
I've seen this crossword clue in Financial Times
A
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Buddy explains!
The straight part
We know that "It’s helpful when a person is out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s helpful when a person is out = answerphone.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 ANSWERPHONE mean?
ANSWERPHONE noun- A telephone containing a built-in device for recording incoming callers’ messages for later retrieval.