Crossword Genius

Appalling? It's impossible to say (11)

Ross

I believe the answer is:

unspeakable

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

'appalling?' is the definition.
(thesaurus)

'impossible to say' is the wordplay.
I cannot really understand how this works, but
'say' could be 'speak' (synonyms) and 'speak' is found in the answer.
The remaining letters 'unable' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'it's' acts as a link.

Can you help me to learn more?

(Other definitions for unspeakable that I've seen before include "dreadful" , "Is impossible pronunciation so" , "Was Marcel Marceau's act so" , "awful?" , "The hardest tongue twisters are" .)

I've seen this clue in The Guardian.
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