Crossword Genius

Astounding cast supplied with sharp content (9)

Ross

I believe the answer is:

startling

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

'astounding' is the definition.
(thesaurus)

'cast supplied with sharp content' is the wordplay.
I cannot quite understand how this works, but
'sharp' could be 'tart' (I've seen this before) and 'tart' is located in the answer.
'cast' could be 'sling' (slinging is a kind of casting) and 'sling' is present in the leftover letters.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for startling that I've seen before include "Producing sudden alarm" , "Rattlings of alarming kind" , "Causing shock" , "Alarming due to rattlings" , "Surprising or alarming" .)

I've seen this clue in The Independent.
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