Picked up boxes one's thrown together (10)
I believe the answer is:
improvised
'thrown together' is the definition.
The definition and answer can be both to do with creating things as well as being past participle verbs.
Maybe you can see a link between them that I can't see?
'picked up boxes one's' is the wordplay.
'picked up' becomes 'improved' (I can't justify this - if you can you should believe this answer much more).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'one's' becomes 'i's' (Roman numeral).
'improved' going around 'is' is 'IMPROVISED'.
Can you help me to learn more?
(Other definitions for improvised that I've seen before include "Extempore" , "Spontaneously created" , "unprepared" .)