Wager — a second in series (4)
I believe the answer is:
beta
'second in series' is the definition.
(second letter of the Greek alphabet)
'wager a' is the wordplay.
'wager' becomes 'bet' (synonyms).
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Second-rate" , "Second Greek letter - beat that" , "less than perfect mark" , "Greek character - not quite top-notch" , "Second highest grade of mark" .)