The most common and most likely answer to this crossword clue is the 2 plus 4 letter word IN TRAY
I
N
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A
Y
Buddy explains!

The straight part
We know that "where mail should be kept" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where mail should be kept = in tray.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 IN TRAY mean?
IN TRAY noun- A desktop organizer; inbox, in-basket.
- (by extension) The amount of new work yet to be done.