Operates buses that won't start (4)
I believe the answer is:
uses
'operates' is the definition.
(I've seen this before)
'buses that won't start' is the wordplay.
'that won't start' means to remove the first letter.
'buses' with its first letter removed is 'USES'.
(Other definitions for uses that I've seen before include "Exercises" , "Puts to a particular purpose" , "Purposes; consumes" , "manipulates" , "Employments" .)