Spur of the moment (9)

I think it could be either:

Which length description are you looking for?:

impromptu

'spur of the moment' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for impromptu that I've seen before include "Unprepared" , "Off-the-cuff" , "On the spur of the moment" , "Piano piece" , "Without rehearsal or preparation" .)

I've seen this clue in The Guardian.

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