Risky transaction with a Greek character (4)
I believe the answer is:
beta
'greek character' is the definition.
(letter from the Greek alphabet)
'risky transaction with a' is the wordplay.
'risky transaction' becomes 'bet' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Second letter of the Greek alphabet" , "less than perfect mark" , "second class grade?" , "Second Greek letter, B" , "Greek character - not quite top-notch" .)