Encumber (6)
'encumber' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for hamper that I've seen before include "Food parcel" , "Hinder, impede" , "Picnic box" , "basket case?" , "basket with a cover" .)
(Other definitions for lumber that I've seen before include "Sawn timber" , "With difficulty move" , "Move heavily; timber" , "Burden with an unwanted task" , "Move awkwardly - unwanted household items" .)