 Clarke akabva apedza mamwe makore matatu ari Commander, achitungamirira Protection Command pakati pa1997 na2000, chinzvimbo chakamuita kuti ave mutariri wekuchengetedzwa kweMadzimambo panguva yekufa kwaDiana, Mambokadzi weWales, achiteverwa nemakore maviri semutevedzeri wemutungamiriri wevashandi kubva 2000 kusvika 2002. Muna June 2002, Clarke akava mukuru weAnti-terrorist Branch, akazobatanidzwa neSpecial Branch kuti aumbe Counter Terrorist Command, basa rakamuisa mukutonga zvakananga kuongororwa kwekurwiswa kweJuly 7pamusoro peLondon uye kuedza kwebhomba kwakakundikana kwaJuly 21, 2005. 