Not a good mark to get (6)
I believe the answer is:
stigma
'not a good mark to get' is the definition.
I can't judge whether this defines the answer.
'not a good mark to get' is the wordplay.
'not' becomes 'sti' (I can't justify this - if you can you should believe this answer much more).
'good' becomes 'g' (abbreviation).
'mark' becomes 'm'.
'to get' is an insertion indicator.
'g'+'m'='gm'
'sti'+'a'='stia'
'stia' enclosing 'gm' is 'STIGMA'.
Can you help me to learn more?
(Other definitions for stigma that I've seen before include "part of plant" , "Mark of infamy or disgrace" , "Bad mark" , "Mark of dishonour" , "A stain as on character" .)