What does the cryptic crossword clue "Reckoning opener's out preparing to bat (8)" mean?

The wordplay uses acrostic. ADDING (synonym="ADDING") + UP (synonym="=preparing to bat, in cricket") = ADDINGUP The answer is ADDINGUP.

Reckoning opener's out preparing to bat (8)

Independent Cryptic #11922 | 4 Across | 2024-12-25 flower