The most common and most likely answer to this crossword clue is the 5 letter word GIRNS
G
I
R
N
S
Buddy explains!

The straight part
We know that "Ugly expressions" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ugly expressions = girns.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 GIRNS mean?
GIRNS verb- To grimace; to snarl.
- To whinge, moan, complain.
- To make elaborate unnatural and distorted faces as a form of amusement or in a girning competition.