What does the cryptic crossword clue "Building of road in US 21 (8)" mean?

The wordplay uses an anagram. anagram of ROADINUS ("road in US 21") [anagram: "Building"] = DINOSAUR The answer is DINOSAUR.

Building of road in US 21 (8)

Guardian Cryptic #29365 | 1 Down | 2024-04-24 flower