Answer enough is a common phrase in English. I won't say figure of speech, because it's not metaphorical or ungrammatical or anything - it just doesn't match patterns that are widely used otherwise.
Enough, here, is a determiner. It means sufficient, not less than is necessary. Determiners often remove the need for an article. Most determiners go before their nouns, but enough is often used after the relevant noun:
Are you man enough for her?
I have money enough for a good meal.
I'm pedant enough to go around correcting people's grammar.
These might all be rephrased:
Are you enough of a man for her?
I have enough money for a good meal.
I'm enough of a pedant to go around correcting people's grammar.
We can see that, for a mass noun, it can just go before or after. For a countable noun, however, or an always-singular noun, if it goes after the noun there's no other words, but if it goes before you need of and the appropriate article.
Thus, the example in your question can be rephrased:
...but the Weasley's looks of disgust were enough of an answer.
Now, there's the question of "enough for what?", but that's a whole other kettle of fish.