The suffering polar bear has become a symbol of a warming world, its plight (BEING) a warning that the clock is ticking.
This sentence is absolutely fine grammatically. There are two closely linked statements here separated by a comma. The second half of the sentence starts with ITS which refers to the subject (the suffering polar bear) of the first half. If BE was present, it would be in the -ING form (being), because it is not the main verb in the sentence.