Set off to arrange dieting (7)
I believe the answer is:
ignited
'set off' is the definition.
(I've seen this before)
'arrange dieting' is the wordplay.
'arrange' indicates anagramming the letters.
'dieting' with letters rearranged gives 'IGNITED'.
'to' acts as a link.
(Other definitions for ignited that I've seen before include "Kindled" , "Sparked off" , "Lit" , "Set alight" , "fired" .)