Stopping short of a second grade marking (10)
I believe the answer is:
abstaining
'stopping short of' is the definition.
I can't tell whether this defines the answer.
'a second grade marking' is the wordplay.
'second' becomes 'b' (B can mean second eg B-side).
'grade marking' becomes 'staining' (I can't explain this - if you can you should believe this answer much more).
'a'+'b'+'staining'='ABSTAINING'
Can you help me to learn more?
(Another definition for abstaining that I've seen is " Refusing to vote".)