The most common and most likely answer to this crossword clue is the 9 letter word UNEXCITED
U
N
E
X
C
I
T
E
D
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 = unexcited.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 UNEXCITED mean?
UNEXCITED adjective- Not feeling excitement or keen interest; placid; bored.
- Not in a state of excitation.