Popular hamper included almost everything (9)
I believe the answer is:
prevalent
'popular' is the definition.
(thesaurus)
'hamper included almost everything' is the wordplay.
'hamper' becomes 'prevent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'included' indicates putting letters inside.
'almost' means to remove the last letter (most of the word but not all of it).
'everything' becomes 'all' (all things).
'all' with its final letter taken away is 'al'.
'prevent' enclosing 'al' is 'PREVALENT'.
Can you help me to learn more?
(Other definitions for prevalent that I've seen before include "Widely accepted" , "Widespread - current" , "Common" , "Commonplace" , "general" .)