Crossword Genius

Is this High German dialect notably sibilant? (5)

Ross

I believe the answer is:

swiss

I'm a little stuck... Click here to teach me more about this clue!

'this high german dialect notably sibilant?' is the definition.
Although both the answer and definition are adjectives, I cannot understand how one could define the other.

'is' is the wordplay.
I cannot quite see how this works, but
'is' is found in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for swiss that I've seen before include "Zurich native" , "Native to Berne" , "Mid-European" , "One from Basle" , "Helvetian" .)

I've seen this clue in the Evening Standard.
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