The most common and most likely answer to this crossword clue is the 8 letter word DEAD-PAN
I've seen this crossword clue in The Times
D
E
A
D
-
P
A
N
Buddy explains!

The straight part
We know that "Impassive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impassive = dead-pan.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 DEAD-PAN mean?
DEAD-PAN noun- A style of comedic delivery in which something humorous is said or done while not exhibiting a change in emotion or facial expression.
- To express (oneself) in an impassive or expressionless manner.
- Deliberately impassive or expressionless.
- Having such a face or look.
- Impassive.
- In a deadpan manner.