Lay down, in a way (6)
(Other definitions for assert that I've seen before include "Swear" , "Maintain formally" , "Declare, state vigorously" , "Aver, maintain" , "Confidently say" .)
(Other definitions for bet that I've seen before include "Money risked on a gamble" , "Wager, gamble" , "22 across will accept one" , "'Stake, wager (3)'" , "Speculation" .)