A clause is a part of a sentence, but it has a subject and verb. It doesn't have to be a complete sentence. eg. "when I get home" is a time clause. A phrase is... well, trickier to explain because the meaning of "phrase" is vague. It can sometimes be used to mean a sentence. As far as grammar goes, a phrase consists of words that form a complete unit but is still less than a clause/sentence. eg. "up on the roof".