Translation:These clothes are dirty.
Just like in English, there are different ways of saying the same thing. While the meaning between "dirty" and "not clean" are the same, they are not the same word. Therefore, "not clean" is not an acceptable answer because the Japanese sentence does not say "not clean" whatsoever.
Yes and no; ふく can translate to either of those in English, such as in this sentence: このふく = "this piece of clothing" / "these clothes". But when translating from English, "clothing" can also be 衣料 （いりょう）in a very general sense of the word, or 衣料品（いりょうひん）when you're talking about it as products.