Appalling? It's impossible to say (11)
I believe the answer is:
unspeakable
'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" .)