The most common and most likely answer to this crossword clue is the 8 letter word SIBILANT
S
I
B
I
L
A
N
T
Buddy explains!
The straight part
We know that "Hissing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hissing = sibilant.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 SIBILANT mean?
SIBILANT noun- A consonant having a hissing sound such as the 's' or 'sh' in 'sash' or 'surge'.
- Characterized by a hissing sound such as the "s" or "sh" in sash or surge.