Wager about a defeat (4)
I believe the answer is:
beat
'defeat' is the definition.
(I know that beat is a kind of defeat)
'wager about a' is the wordplay.
'wager' becomes 'bet' (synonyms).
'about' is an insertion indicator.
'bet' enclosing 'a' is 'BEAT'.
(Other definitions for beat that I've seen before include "club" , "patrol route?" , "Master" , "Musical rhythm" , "Worst - best" .)