What does the cryptic crossword clue "Rub out nonsense — that’s a mistake! (4)" mean?

The definition is "mistake!". The wordplay uses a hidden word. BISH (deletion) = BISH; definition: "mistake!" The answer is BISH.

Rub out nonsense — that’s a mistake! (4)

Guardian Cryptic #29155 | 1 Down | 2023-08-22 flower