The most common and most likely answer to this crossword clue is the 13 letter word RAT-A-TAT-TAT
I've seen this crossword clue in Financial Times
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More clues leading to the result RAT-A-TAT-TAT
Buddy explains!
The straight part
We know that "Postman’s knock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case postman’s knock = rat-a-tat-tat.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.
Crossword Answer definitions
RAT-A-TAT-TAT noun- A series of short, sharp taps, especially made by knocking on a door, beating a drum, or firing an automatic weapon
- A series of short, sharp taps, especially made by knocking on a door, beating a drum, or firing an automatic weapon