Short time with hesitation giving impetus (8)
I believe the answer is:
momentum
'impetus' is the definition.
(I've seen this before)
'short time with hesitation' is the wordplay.
'short time' becomes 'moment' (moment is a kind of short time).
'with' means one lot of letters go next to another.
'hesitation' becomes 'um' (um is said in hesitation).
'moment'+'um'='MOMENTUM'
'giving' acts as a link.
(Other definitions for momentum that I've seen before include "Forward impetus" , "Driving force, impetus" , "Drive" , "Impetus from motion" , "Impulse" .)