Prophetess is reflected by lake (5)
I believe the answer is:
sibyl
'prophetess' is the definition.
(I've seen this before)
'is reflected by lake' is the wordplay.
'reflected' says the letters should be written in reverse.
'lake' becomes 'l'.
'is' reversed gives 'si'.
'si'+'by'+'l'='SIBYL'
(Other definitions for sibyl that I've seen before include "Seer in classical times" , "Female seer" , "Greek oracle" , "Woman mouthpiece of a god, uttering prophecies" , "Seeress; Mrs. Fawlty" .)