Sentence:  Today , in the era of the integrated musical when an individual song must contribute to the over-all development of the show , it <WSD>is</WSD> understandable that a song , no matter how excellent it may be on its own terms , is cut out because it does not perform the function required of it . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)