Maybe warning notice has a bad fault (6)
I believe the answer is:
advice
'maybe warning' is the definition.
(I know that Admonition is a type of advice)
'notice has a bad fault' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'has' says to put letters next to each other.
'a bad fault' becomes 'vice' (I've seen this before. I am not sure about the 'bad' bit.).
'ad'+'vice'='ADVICE'
Can you help me to learn more?
(Other definitions for advice that I've seen before include "Intelligence" , "Opinion offered as to future action" , "Proposal for an appropriate course of action" , "Suggestion (invited or otherwise)" , "Formal notice" .)