Tip of an iceberg is it a metaphor or a cliche?

Although things can be metaphors and cliches, this one is probably best described as cliche-if you were say that "something is like the tip of the iceberg" that would be a simile, if you were say "something is the tip of the iceberg" that would be a metaphor. But as it's a well used phrase whatever it can be generally thought of as a cliche.