Forecaster lives up near far end of Hull (5)
I believe the answer is:
'forecaster' is the definition.
(I know that sibyl is a type of fortune teller)
'lives up near far end of hull' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'near' becomes 'by' (near means close by).
'far end of' says to take the final letters.
The last letter of 'hull' is 'l'.
'is' backwards is 'si'.
(Other definitions for sibyl that I've seen before include "prophet" , "Female seer" , "Greek oracle" , "Seer in classical times" , "One seeing future" .)