Without preparation, rascal hosts formal - oddly true (9)

I believe the answer is:

impromptu

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'without preparation' is the definition.
(I've seen this before)

'rascal hosts formal oddly true' is the wordplay.
'rascal' becomes 'imp' (synonyms).
'hosts' is an insertion indicator (inserted letters are welcomed or hosted).
'formal' becomes 'prom' (I've seen this in another clue).
'oddly true' becomes 'tu' (I can't justify this - if you can you should give a lot more credence to this answer).
'imp' going around 'prom' is 'impromp'.
'impromp'+'tu'='IMPROMPTU'

Can you help me to learn more?

(Other definitions for impromptu that I've seen before include "Without much planning" , "Piano piece" , "Unplanned; composition" , "With little or no preparation or forethought" , "Unscripted" .)

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

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