Set off crash dieting (7)
I believe the answer is:
ignited
'set off' is the definition.
(I've seen this before)
'crash dieting' is the wordplay.
'crash' is an anagram indicator.
'dieting' anagrammed gives 'IGNITED'.
(Other definitions for ignited that I've seen before include "Sparked off" , "Gin diet (anag.)" , "Set on fire" , "Kindled" , "Set alight" .)