Set out (5)
'set out' is the definition.
(synonyms)
This is all the clue.
(Other definitions for begin that I've seen before include "Commence" , "Monachem --" , "Launch" , "Israeli prime minister who shared the Nobel Peace Prize with President Sadat" , "Make a start" .)
'set out' is the definition.
(sallying forth is a kind of setting out)
This is the entire clue.
(Another definition for sally forth that I've seen is " Go out".)