Much enthusiasm about brother's wild horse (5)
I believe the answer is:
zebra
'wild horse' is the definition.
(I've seen this before)
'much enthusiasm about brother's' is the wordplay.
'much enthusiasm' becomes 'zea' (I can't explain this - if you can you should believe this answer much more).
'about' means one lot of letters goes inside another.
'brother' becomes 'br' (abbreviation in titles of monks).
'zea' enclosing 'br' is 'ZEBRA'.
Can you help me to learn more?
(Other definitions for zebra that I've seen before include "Animal like striped pony" , "Striped creature" , "Striped hoofed beast" , "Black and white striped animal" , "Striped member of the horse family" .)