Impetus bringing hesitation after a little while (8)
I believe the answer is:
momentum
'impetus' is the definition.
(thesaurus)
'hesitation after a little while' is the wordplay.
'hesitation' becomes 'um'.
'after' means one lot of letters go next to another.
'a little while' becomes 'moment' (I've seen this before).
'um' after 'moment' is 'MOMENTUM'.
'bringing' acts as a link.
(Other definitions for momentum that I've seen before include "Corbyn supporters" , "Drive" , "Impetus gained by movement, maybe downhill" , "driving power" , "Forward impetus" .)