For a short time impetus is produced without any hesitation (6)
I believe the answer is:
moment
'for a short time' is the definition.
(moment is a kind of short time)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for moment that I've seen before include "Brief period of time" , "tick" , "Jiffy" , "Brief second" , "Occasion" .)