The most common and most likely answer to this crossword clue is the 6 letter word GLASSY
I've seen this crossword clue in The Daily Telegraph
G
L
A
S
S
Y
Buddy explains!

The straight part
We know that "Expressionless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expressionless = glassy.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 GLASSY mean?
GLASSY noun- Glass marble.
- Of or like glass, especially in being smooth and somewhat reflective.
- Including a lot of glass.
- Dull; expressionless.
- (of water) Lacking any chop; smooth and mostly flat.