Respond to an order for a crate (5)
I believe the answer is:
react
'respond' is the definition.
(to react to something is to respond to it)
'an order for a crate' is the wordplay.
'an order for' indicates an anagram.
'crate' anagrammed gives 'REACT'.
'to' is the link.
(Other definitions for react that I've seen before include "Change chemically" , "Trace (anag)" , "Act as opponent" , "Respond emotionally" , "What a chemical might do" .)