Assist criminal in a wager (4)
I believe the answer is:
abet
'assist criminal' is the definition.
'abet' can be an answer for 'assist' (abetting is a kind of assisting). I am not certain of the 'criminal' bit.
'a wager' is the wordplay.
'wager' becomes 'bet' (synonyms).
'a'+'bet'='ABET'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for abet that I've seen before include "Encourage (wrongdoing)" , "Criminally endorse" , "Assist in some nefarious activity" , "Make good" , "Aid and assist" .)