Crossword Genius

Tough enthralling vault a promising sign (4,4)

Ross

I believe the answer is:

good omen

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

'a promising sign' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'tough enthralling vault' is the wordplay.
'tough' becomes 'goon' (synonyms).
'enthralling' is an insertion indicator.
'vault' becomes 'dome' (I've seen this in another clue).
'goon' enclosing 'dome' is 'GOOD OMEN'.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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