If not, to Nero (4)
I believe the answer is:
nisi
'if not to nero' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nisi that I've seen before include "(Of decree) valid if certain conditions are met" , "Kind of divorce decree" , "(Of decree) taking effect when conditions are met" , "Unless - in Latin" , "'De mortuis nil .... bonum.'" .)