20A Think of a synonym for "severity" and remove a letter- the answer is apparatus used with horses.
24A "Hoover' is another example of the answer.
33A A classical architect- the answer is an anagram of "retail" plus "B".
13D A short word used to express disgust inserted in a four letter word for a dance, after an abbreviation for British.
30D Shorten "zombies", then solve the anagram for a type of African witchcraft/religion.
36D A common abbreviation for a day of the week, and move the "U" for "united".