What does the cryptic crossword clue "Boatman under duty to provide conveyance (4)" mean?

The wordplay uses a charade (building blocks). TAX (synonym="duty") + I (synonym="Boatman") = TAXI The answer is TAXI.

Boatman under duty to provide conveyance (4)

Guardian Cryptic #29541 | 5 Down | 2024-11-15 flower