What does the cryptic crossword clue "Hearing what you can do on line, one's enthralled (4)" mean?

The wordplay uses a homophone (sounds like). Homophone of “surf”, as in “surfing the web” The answer is SERF.

Hearing what you can do on line, one's enthralled (4)

Guardian Cryptic #29247 | 26 Across | 2023-12-07 flower