Pass with two good marks and a bad one (7)
I believe the answer is:
abstain
'pass' is the definition.
(I've seen this before)
'two good marks and a bad one' is the wordplay.
I cannot really understand how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
'a' is within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'with' acts as a link.
Can you help me to learn more?
(Other definitions for abstain that I've seen before include "Refrain (from drink)" , "Don't vote" , "Pass" , "Formally decline to vote" , "Do without, as in lent" .)