Possible Clues:

• 'Crikey!' (that's what it says here), 'a sea-girt piece of land!'
• Mediterranean island, chief city Ajaccio
• Company car is wrecked in island
• First clue - one across - virtually about an island in the Med
• Island of Seneca's exile
• Chevy model
• Napoleon Bonaparte was born on this French island
• Where Napoleon Bonaparte delivered source of drug, having admitted the strain of office work
• Island thus sheltered by marine growth, for the most part
• Island of W Italy, capital Ajaccio