Demand wager about container (6)
I believe the answer is:
basket
'container' is the definition.
(basket is a kind of container)
'demand wager about' is the wordplay.
'demand' becomes 'ask' ('ask' can be a synonym of 'demand').
'wager' becomes 'bet' ('bet' can be a synonym of 'wager').
'about' indicates putting letters inside.
'ask' going within 'bet' is 'BASKET'.
(Other definitions for basket that I've seen before include "Wicker carrier" , "Hamper" , "Shopping holder" , "Container made of interwoven cane or wire" , "it contains" .)