Impertinent assurance in manner vehemently denied (5)

I believe the answer is:

nerve

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

'impertinent assurance' is the definition.
'nerve' can be an answer for 'assurance' (thesaurus). I am not certain of the 'impertinent' bit.

'in manner vehemently denied' is the wordplay.
'in' says the answer is hidden in the clue.
'NERVE' is hidden within 'manner vehemently denied'.

Can you help me to learn more?

(Other definitions for nerve that I've seen before include "Courage, coolness in danger" , "Cool courage" , "Sensory organ; bottle" , "Valour, pluck" , "Fearlessness" .)

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

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