Prophetess is reflected by lake (5)
I believe the answer is:
Here is my best explanation:
'prophetess' is the definition.
(I've seen this before)
'is reflected by lake' is the subsidiary indication.
'reflected' says the letters should be written backwards.
'lake' becomes 'l'.
'is' backwards is 'si'.
(Other definitions for sibyl that I've seen before include "prophet", "Female seer", "Ancient seer - prophetess", "Seer in classical times", "Woman mouthpiece of a god, uttering prophecies", "Seeress; Mrs. Fawlty", "Prophetess", "Charmer", "Greek oracle" and "One seeing future".)