Under no circumstances! (3,2,4,4)
I believe the answer is:
not on your life
'under no circumstances' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for not on your life that I've seen before include "Like someone else's autobiography" , "No way!" , "There's no chance" , "On no account whatsoever" , "Emphatically, no!" .)