On the /help menu it shows /clear <player>. But just "/clear" works so it should be /clear [player]

/clear <player> [item] [data] [maxCount] [dataTag]
Should mean that <player> is required. But it's not, just:

1. GO IN TO GAME
2. type "/clear", IT WORKS

So it should be:
/clear [player] [item] [data] [maxCount] [dataTag]