Two bad marks left in homework exercise? That's nothing! (6,5)
I believe the answer is:
diddly squat
'nothing' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'two bad marks left in homework exercise?' is the wordplay.
I cannot quite see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'exercise?' could be 'squat' (squat is a kind of exercise) and 'squat' is found within the answer.
'bad marks' could be 'dd' (E - a bad exam grade - written twice) and 'dd' is found within the answer.
'homework' could be 'diy' (DIY is work done on your home) and 'diy' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?