There have been reports of the Clip not playing well with cards that are faster than Class 4 in speed. You might try re-formatting the card using the SD Association’s formatter tool, which sometimes can help with card issues. https://www.sdcard.org/downloads/formatter_3/ Do note that your card needs to be in FAT32 format for the Clip.
Formatting using either MTP mode, or the format option the Menu is probably the best way to go. If the player still refuses to use this card, you should consider replacing it with a class, 4 as suggested.
It might be a defective or counterfeit card. If defective your dealer (or Sandisk) shold replace it under warranty. If counterfeit, I guess you’re hosed (screwed or out of luck).
You can download a free utility, h2testw to check it.
Yep, it is the shame that the Clips can’t handle cards faster than Class 4 (often), and that the products specs don’t note this, at this point . . . . But a good reason why I rty to only buy computer products that I know I can return, if there is an issue with compatibility or otherwise.