If you do a few define: operator searches and examine the source code of the pages in the results, you can find several ways to create a glossary page that will be included in those searches. The key seems to be to clearly identify the information as a glossary or dictionary, and to structure the information within your HTML mark-up in ways that make it simple to parse.
The only one I have made is targeted at a very small niche and it doesn't have to compete against many other glossaries, so I can't say what works best. I made a page that included several paragraphs of text in addition to the glossary, which I intended to add quality and relevance. The more robust definitions seem to rank better, but that's from a very limited sample.