The solid portion of a comet is a phrase.
En grammar, a clause and a phrase are both parts of a sentence but a clause is considered a grouping of words which contain both a subject and a verb. A phrase on the other hand is a grouping of words that does not contain a subject and a verb.
In the example given above, the phrase, 'The solid portion of a comet', although it contains a subject, it lacks a verb, therefore it is not a clause.