A bet on who might win the war? (5)
I believe the answer is:
wager
'a bet on who' is the definition.
'wager' can be an answer for 'a bet' ('wager' can be a synonym of 'bet'). I am not sure about the rest of the definition.
'win the war?' is the wordplay.
'win' becomes 'w' (abbreviation in sport).
'the war?' becomes 'ager' (I can't justify this - if you can you should give a lot more credence to this answer).
'w'+'ager'='WAGER'
'might' is the link.
Can you help me to learn more?
(Other definitions for wager that I've seen before include "act for "The Gamblers"?" , "Punt" , "Risk; stake" , "stake required" , "better to put this on?" .)