1) Lionel, Duke of Clarence, (1338-1368), son of Edward III, poisoned while visiting Italy.
2) Thomas, Duke of Clarence, (1388-1421), son of Henry IV, killed stupidly leading his troops in battle against the French (the archers hadn't arrived but he ordered the attack anyway).
3) George, Duke of Clarence, (1449-1478), younger brother of Edward IV, supposedly drowned in a butt of malmsey wine on his brother's orders.
5) Albert, Duke of Clarence, (1864-1892), the future George V's elder brother, regarded by his family as feeble-minded and not excessively mourned when he died early.
Also there is the Duke of Clarence who never was, Lord Guilford Dudley (1536-1544) who was married to Lady Jane Grey, briefly queen of England in 1543. They were both executed the following year, but the original plan was for him to be made Duke of Clarence if she had managed to hang onto the throne.
The only slightly successful one was 4) William, Duke of Clarence, (1765-1837) who lived longer than any two of the others put together and ended up as King William IV; probably the least memorable of the Hanoverian kings. But sometimes obscurity is a blessing.