Jack often comes after discarded bits and pieces (6)
I believe the answer is:
lumber
'bits and pieces' is the definition.
(lumber can mean clutter or bits and pieces)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lumber that I've seen before include "articles in storage" , "Sawn timber" , "Move in a heavy way, with partly treated wood" , "Wood cut and prepared as building material" , "With difficulty move" .)