Boxes and reacts badly (6)
I believe the answer is:
crates
'boxes' is the definition.
(crate is a kind of box)
'reacts badly' is the wordplay.
'badly' is an anagram indicator.
'reacts' is an anagram of 'CRATES'.
'and' is the link.
(Other definitions for crates that I've seen before include "Jalopies" , "Boxes for shipping goods" , "Packing cases" , "Rough wooden boxes" .)