Ignorant schoolmarm is penniless (4)
I believe the answer is:
rude
'ignorant' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rude that I've seen before include "'Bad-mannered, discourteous (4)'" , "Impolite - simple" , "Base" , "Impolite - obscene" , "Lacking civility" .)