Greek character risked ace (4)
I believe the answer is:
beta
'greek character' is the definition.
(letter from the Greek alphabet)
'risked ace' is the wordplay.
'risked' becomes 'bet' (eg to bet money is to risk it).
'ace' becomes 'a' (abbreviation).
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "It's next after alpha" , "Second Greek letter, B" , "In a constellation, a star second in brightness" , "Letter before gamma" , "Second-rate" .)