Sibyl or Cassandra, say (4)
I believe the answer is:
seer
I believe this is a double definition.
'sibyl' is the first definition.
(I've seen this in another clue)
'cassandra say' is the second definition.
(I know that Cassandra is a prophetess)
(Other definitions for seer that I've seen before include "Oracle or clairvoyant" , "Distinguishing quality" , "One having visions of the future" , "Sage with 20ac" , "One has unusual foresight" .)