Cast down with a single blow (5)
I believe the answer is:
abash
'cast down' is the definition.
(thesaurus)
'with a single blow' is the wordplay.
'with' says to put letters next to each other.
'a single' becomes 'a'.
'blow' becomes 'bash' (bash is a kind of blow**).
'a' put next to 'bash' is 'ABASH'.
(Other definitions for abash that I've seen before include "Humiliate" , "Bring shame on" , "put one to shame" , "Cause embarrassment" , "disgrace" .)