Assist a wager (4)
I believe the answer is:
abet
'assist' is the definition.
(abetting is a kind of assisting)
'a wager' is the wordplay.
'wager' becomes 'bet' ('bet' can be a synonym of 'wager').
'a'+'bet'='ABET'
(Other definitions for abet that I've seen before include "Urge on, help" , "Criminally endorse" , "Assist illegally" , "Aid and assist" , "Encourage (offence)" .)