Well, I have trawled the net (including the excellent "The Peerage.com" but cannot find an Earl of Usk anywhere.
Maybe you misheard the name "The Earl of Wessex" (Prince Edward.)?
Here is the Royal Family Order of Succession,(from The Royal Family Website) and as you will see,there is no Earl of Usk.
At No 20,in the list,there IS The Earl of Ulster.Might it be him?
Alexander Patrick Gregers Richard Windsor, Earl of Ulster
He is the son of Richard,Duke of Gloucester (the Queen's Cousin)
THE LINE OF SUCCESSION
Sovereign
1. The Prince of Wales
2. Prince William of Wales
3. Prince Henry of Wales
4. The Duke of York
5. Princess Beatrice of York
6. Princess Eugenie of York
7. The Earl of Wessex
8. Viscount Severn
9. The Lady Louise Windsor
10. The Princess Royal
11. Mr. Peter Phillips
12. Miss Zara Phillips
13. Viscount Linley
14. The Hon. Charles Armstrong-Jones
15. The Hon. Margarita Armstrong-Jones
16. The Lady Sarah Chatto
17. Master Samuel Chatto
18. Master Arthur Chatto
19. The Duke of Gloucester
20. Earl of Ulster
21. Lord Culloden
22. The Lady Davina Lewis
23. The Lady Rose Windsor
24. The Duke of Kent
25. The Lady Amelia Windsor
26. The Lady Helen Taylor
27. Master Columbus Taylor
28. Master Cassius Taylor
29. Miss Eloise Taylor
30. Miss Estella Taylor
31. The Lord Frederick Windsor
32. The Lady Gabriella Windsor
33. Princess Alexandra, the Hon. Lady Ogilvy
34. Mr. James Ogilvy
35. Master Alexander Ogilvy
36. Miss Flora Ogilvy
37. Miss Marina Ogilvy
38. Master Christian Mowatt
39. Miss Zenouska Mowatt
40. The Earl of Harewood