What does the cryptic crossword clue "Tonga is built around a capital city (8)" mean?

The definition is "city". The wordplay uses an anagram. anagram of TONGA + IS + A = SANTIAGO; definition: "capital city" The answer is Santiago.

Tonga is built around a capital city (8)

Telegraph Prize Cryptic #3041 | 24 Across | 2020-02-02 flower