Lose impetus (3,3,2,5)
I believe the answer is:
run out of steam
'lose impetus' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for run out of steam that I've seen before include "Have no motive force left" , "like Awdry's Thomas?" , "Lack the energy to continue" , "Lose enthusiasm" , "Lose impetus" .)