Gamble on a letter from Greece (4)
I believe the answer is:
beta
'letter from greece' is the definition.
(letter from the Greek alphabet)
'gamble on a' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "sort of test" , "software under test" , "second class grade?" , "In a constellation, a star second in brightness" , "character in Marathon" .)