Gamble with a Greek character (4)
I believe the answer is:
beta
'greek character' is the definition.
(letter from the Greek alphabet)
'gamble with a' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling).
'with' says to put letters next to each other.
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Old character" , "In a constellation, a star second in brightness" , "final test" , "Second-rate" , "Second Greek letter - beat that" .)