What does the cryptic crossword clue "Old style piece found in treasure trove (5)" mean?

The definition is "Old style". The wordplay uses a hidden word. Hidden in "old style piece found in treasuRE TROve" The answer is RETRO.

Old style piece found in treasure trove (5)

Guardian Cryptic #28902 | 19 Across | 2022-10-31 flower