Learn Russian: Он дал ясный ответ на вопрос.

In English we say "an answer to a question", which might tempt you to use the Dative case. However, in Russian, the noun ответ (answer) requires the preposition на followed by the Accusative case: ответ на вопрос.

How would we apply our current lesson's grammar to say "He gave such a clear answer that everyone understood"?

Because ответ (answer) is a masculine noun, you need the masculine form такой (such). The sentence would become: Он дал такой ясный ответ, что все поняли. You use такой rather than так here because it modifies a noun phrase.

Could I use понятный instead of ясный for "clear"?

Yes, понятный ответ (an understandable/comprehensible answer) works perfectly here. Ясный literally means "clear" (like a clear sky) and is used metaphorically for something that is unambiguous and obvious. Понятный specifically means that something makes logical sense to the mind.

What would it mean if we used the imperfective verb давал instead of дал?

Using the perfective дал focuses on a single, completed event: he gave this specific answer once. If you used давал, it would mean "he was giving" (an ongoing process in the past) or "he used to give" (a repeated habit).