Apart from the different spelling in British (defence) and American English (defense), there are some interesting points to be mentioned about the pronunciation of the word defence. The Oxford Learner’s Dictionary only mentions the pronunciation [dɪˈfens], both for British and American English, listen here. However, in practice, you will also encounter the variant [dəˈfens], as listed in Longman’s Pronunciation Dictionary. More notably, if you ever watch sports on American Television, there is a good chance you will hear defence pronounced as [‘dɪ:fens], with the stress on the first syllable. [‘dɪ:fens] is included in Longman’s Pronunciation Dictionary as an alternative American pronunciation. From my experience and discussions such as this one, the variant [‘dɪ:fens] seems to be used exclusively in sports.