What does the cryptic crossword clue "Not ready to pick up rein, having been thrown (6)" mean?

The definition is "Not ready". The wordplay uses an anagram. anagram of UP + REIN, = UNRIPE; definition: "Not ready" The answer is UNRIPE.

Not ready to pick up rein, having been thrown (6)

Daily Mail Cryptic #16743 | 21 Down | 2022-01-04 flower