Responds by breaking up crates (6)
I believe the answer is:
reacts
'responds' is the definition.
('react' can be a synonym of 'respond')
'breaking up crates' is the wordplay.
'breaking up' is an anagram indicator.
'crates' anagrammed gives 'REACTS'.
'by' acts as a link.
(Other definitions for reacts that I've seen before include "Retaliates" , "Responds to stimulus (6)" , "Responds to an impulse" , "Acts in response to something" .)