Prediction by a Greek character (4)
I believe the answer is:
beta
'greek character' is the definition.
(letter from the Greek alphabet)
'prediction by a' is the wordplay.
'prediction' becomes 'bet' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Greek letter -- second in order of importance" , "Greek character - not quite top-notch" , "final test" , "2nd letter of Greek alphabet" , "Old character" .)