Learn Croatian: Vijeće UN-a prihvatilo je rezoluciju. - The UN council adopted the resolution.

This is the genitive case, meaning "of the UN". In Croatian, capitalized acronyms are generally treated as regular masculine nouns. To show their case, we attach standard masculine endings using a hyphen, like UN-a (genitive) or UN-u (locative).

Why is the verb "prihvatilo" in the neuter form if the acronym UN acts as a masculine noun?

The verb must agree with the grammatical subject of the sentence. Here, the subject is Vijeće (council), which is a neuter noun. The acronym UN-a is just modifying it, so it doesn't affect the verb's gender.

Why isn't the enclitic "je" placed immediately after the first word, "Vijeće"?

Enclitics usually take the second position in a sentence, but "second position" can mean the second word OR the second phrase. Vijeće UN-a acts as a single grammatical phrase. You could write Vijeće UN-a je prihvatilo rezoluciju, but placing je after the verb (prihvatilo je) is also a perfectly natural way to keep the subject phrase intact.

Is "rezoluciju" a regular feminine noun in the accusative case?

Yes. Like most international words ending in "-tion" in English, it ends in -cija in Croatian (rezolucija). These are regular feminine nouns, so the accusative case always takes the standard -u ending.