Maybe sly about a bit of a fib, the witch! (5)
I believe the answer is:
sibyl
'witch' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sibyl that I've seen before include "Ancient Roman prophetess" , "Ancient seer - prophetess" , "Greek prophetess" , "Charmer" , "Woman mouthpiece of a god, uttering prophecies" .)