Get rolling (5)
I believe the answer is:
'get rolling' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for begin that I've seen before include "Institute" , "Make a start" , "Clandestine, furtive" , "Israeli prime minister who shared the Nobel Peace Prize with President Sadat" , "former Israeli Nobel peace prize winner" .)