It's bad form not to (4)
I believe the answer is:
from
'not to' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'it's bad form' is the wordplay.
'it's bad' indicates anagramming the letters.
'form' anagrammed gives 'FROM'.
Can you help me to learn more?
(Other definitions for from that I've seen before include "Springing out of" , "Beginning at; out of" , "Sent by" .)