It depends on your background I guess. If you have done an internship in tax then they will probably expect a few concepts with which you are familiar but the majority of people are unlikely to have an accounting background/degree as this is what they are going to teach you. 10 minutes is not long at all especially once you account for the intro and conclusion. My advice would be:
> pick a company that genuinely interests you and is not a huge company that the partner is likely to know all about (such as one of the supermarkets)
> concentrate on the bigger picture as opposed to tax exclusively
> structure your presentation and try and break it down into 4-6 headings so that the partner has a clear understanding of what you have said
For instance take Dyson vacuum cleaners. In order to help it grow/ develop there are several things you could do: increase the product range (eg new air blade hand driers), increase production, increase exports/ enter new markets, take over other firms, outsource production, get a listing on AIM/FTSE and grow organically through further investment in production/ R&D.
They want to see that you have thought about whatever company you choose to present on (mine was not on dyson). Also if you can, try and actually present it to someone with a background in business. This will not only give you practice at presenting it and timing, but make you more confident. Get them to ask you questions so you get experience of thinking on your feet and can anticipate likely questions: 'what do you think the biggest challenge is facing dyson?', 'what happens when it looses its patent?' etc, etc
hope this helps, good luck