Sentence:  Private-equity nabobs bristle at being dubbed mere financiers. Piling debt onto companies' balance-sheets is only a small part of what leveraged buy-outs are about, they insist. Improving the workings of the businesses they take over is just as <WSD>core</WSD> to their calling, if not more so. Much of their pleading is public-relations bluster. 
Meaning of the word marked with <WSD> tags:  The most important part of a thing; the essence.