Learn Dutch: Het is nu wat bewolkt, maar het zal straks opklaren. - It is somewhat cloudy now, but it will clear up later.

You are right that wat is the question word 'what'. However, when placed as an adverb directly before an adjective like bewolkt (cloudy), it acts as a degree modifier meaning 'somewhat', 'a little', or 'a bit'. So wat bewolkt means 'somewhat cloudy'.

Why is the time word nu placed before wat bewolkt?

This follows the general Dutch word order guideline of 'Time, Manner, Place' (often called TMP). The time adverb nu (now) generally comes before the descriptive state or manner, which in this case is wat bewolkt.

Why doesn't the word order change after maar?

The word maar (but) is a coordinating conjunction. This means it connects two independent main clauses without messing with their internal word order. If the sentence used a subordinating conjunction instead, the verb zal would have been pushed to the end of the sentence.

Is opklaren a separable verb? Why isn't it separated here?

Yes, opklaren (to clear up) is a separable verb made of op + klaren. However, it is not separated here because it is paired with the auxiliary verb zal (will). When an auxiliary verb takes the second position in a clause, the main verb is pushed to the very end as a full, attached infinitive.