I hate interview questions like that. I also hate the ones where you're asked about a time when you went beyond your call of duty, or when you took initiative to do something.
To be honest, there are probably a few examples where you've worked under pressure... and if not, you can make one up. For example, where I work (Asda), we have regular inspections, and because the managers are so useless they're always caught off-guard until the day before the inspection If you're working the day before an inspection a shift is like to entail being rushed off your feet cleaning, tidying, working stock and suchlike, with little chance of getting the breaks you're entitled to... and you really have to perform, otherwise impatience gets the better of the managers (because it's their jobs on the line for once). Also, because they're running all over the place, it sometimes takes someone (i.e. you) to take some leadership and try and organise the people you're working with or who are working around you. Examples like this are good because they're a type of pressure that you're subjected to despite not having had any particular role in causing it (as opposed to revision cramming, for example).