The car jerked and upset the boxes (6)
I believe the answer is:
crates
'boxes' is the definition.
(crate is a kind of box)
'the car jerked and upset' is the wordplay.
'jerked' is an anagram indicator (I've seen 'jerk' mean this).
'and' says to put letters next to each other.
'upset' becomes 'tes' ('set' written backwards or 'up').
'car' with letters rearranged gives 'cra'.
'cra'+'tes'='CRATES'
'the' is the link.
(Other definitions for crates that I've seen before include "Boxes for shipping goods" , "Rough wooden boxes" , "Jalopies" , "Packing cases" .)