Wager, accompanied by a letter (4)
I believe the answer is:
beta
'letter' is the definition.
(beta is a kind of letter of the alphabet)
'wager accompanied by a' is the wordplay.
'wager' becomes 'bet' (synonyms).
'accompanied by' means one lot of letters go next to another.
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Character between alpha and gamma" , "Second Greek letter - beat that" , "It's next after alpha" , "final test" , "character in Marathon" .)