Originally Posted by mr dave

first off - new or used guitar? if it was new and shipped like that - return it.

Originally Posted by mr dave

otherwise - i'd just use an exacto knife and a nail file - on the nut. unless you've got a high end model with a compensated nut (like Earvana or Buzz Feiten systems) but judging from the pic and craftsmanship, you don't, it's likely just a misformed chunk of plastic.

actually a closer look at the picture and the curve in the nut makes it look like it was removed and put in again backwards (but that might also be due to lighting). either way, it's not a huge deal and not something i'd consider serious enough to take to a luthier, especially if it's not a $1000+ guitar. just make sure to keep checking to see if the piece fits properly before you start whittling too much of it away.