The most common and most likely answer to this crossword clue is the 4 letter word TUTS
T
U
T
S
Buddy explains!
The straight part
We know that "Expressions of disapproval" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expressions of disapproval = tuts.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 TUTS mean?
TUTS verb- To make a tut tut sound of disapproval.
- To work by the piece; to carry out tut-work.