What does the cryptic crossword clue "Written in past, one agenda ancient (5-3)" mean?

The definition is "ancient". The wordplay uses a hidden word. Hidden in "written in paST, ONE AGEnda ancient" The answer is Stone-age.

Written in past, one agenda ancient (5-3)

Telegraph Prize Cryptic #3054 | 10 Across | 2020-05-03 flower