The most common and most likely answer to this crossword clue is the 3 letter word OCH
I've seen this crossword clue in The Guardian
O
C
H
More clues leading to the result OCH
Buddy explains!
The straight part
We know that "A Scot’s impatient expression" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a scot’s impatient expression = och.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 OCH mean?
OCH interjection- General interjection of confirmation, affirmation, and often disapproval.
- An expression of anger, frustration, surprise