Wager on a Greek letter (4)
I believe the answer is:
beta
'greek letter' is the definition.
(letter from the Greek alphabet)
'wager on a' is the wordplay.
'wager' becomes 'bet' (synonyms).
'on' says to put letters next to each other.
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "testing stage" , "Second-class" , "less than perfect mark" , "character in Marathon" , "second class grade?" .)