What does the cryptic crossword clue "Working on self-confidence finally failing, a sickener (6)" mean?

The wordplay uses a charade (building blocks). POIS (synonym="POIS") + ON (synonym="working") = POISON The answer is POISON.

Working on self-confidence finally failing, a sickener (6)

Guardian Cryptic #29315 | 9 Across | 2024-02-26 flower