There's nothing that won't go into the bag (7)
I believe the answer is:
holdall
'the bag' is the definition.
(holdall is a kind of bag)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for holdall that I've seen before include "Capacious bag or basket" , "A large soft travelling bag - stuff everything in" , "Large case, with handles" , "Large bag (no need for another?)" , "Large carrying bag" .)