Embarrassing marks, when you suck at subject you like? (9)

I believe the answer is:

lovebites

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'embarrassing marks when you suck' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'at subject you like?' is the wordplay.
'at' means one lot of letters go next to another.
'subject you' becomes 'bites' (I can't explain this - if you can you should give a lot more credence to this answer).
'like?' becomes 'love' (loving is a kind of liking).
'bites' put after 'love' is 'LOVEBITES'.

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I've seen this clue in The Independent.

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