The most common and most likely answer to this crossword clue is the 6 letter word SIZZLE
S
I
Z
Z
L
E
Buddy explains!

The straight part
We know that "hissing sound" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hissing sound = sizzle.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 SIZZLE mean?
SIZZLE noun- The sound of water hitting a hot surface
- Zing, zip, or pizazz; excitement.
- To make the sound of water hitting a hot surface.
- To be exciting or dazzling.