A tangerine is a fictional fruit created by Steven Moffat. The unoriginal hack created a fruit that is essentially an orange, but smaller. Pff, what kind of imagination is that? It's so bloody stupid that it couldn't possibly ever exist in reality. Even The Doctor hates them.
Santa Claus used to give lumps of coal to unsuspecting kiddies, but now Morefat decided that this completely non-existent, stupid waste of creativity is what Sandy Cheeks give all the naughty Time Lads for Christmas.
If you find a tangerine in your house or somewhere about your day, the chances are likely that YOU ARE DYING and should probably attempt to wake up.