The setter, backing what setters do, set out (6)
I believe the answer is:
embark
'set out' is the definition.
(I've seen this before)
'the setter backing what setters do' is the wordplay.
'the setter' becomes 'me' (the person who set this crossword).
'backing' says the letters should be written in reverse.
'what setters do' becomes 'bark' (setter is a type of dog).
'me' backwards is 'em'.
'em'+'bark'='EMBARK'
(Other definitions for embark that I've seen before include "Go on board ship or plane" , "Join ship" , "board production" , "get on board" , "Set out (on an enterprise)" .)