Hum and haw (6)
I believe the answer is:
falter
'hum and haw' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for falter that I've seen before include "Hesitate, begin to give way" , "Hesitate, be uncertain" , "Hesitate, waver" , "Be hesitant" , "Lose momentum" .)