Treated mania left in brutal person (6)
I believe the answer is:
animal
'brutal person' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'treated mania left' is the wordplay.
'treated' indicates an anagram.
'left' becomes 'l' (common abbreviation).
'mania' with letters rearranged gives 'anima'.
'anima'+'l'='ANIMAL'
'in' is the link.
Can you help me to learn more?
(Other definitions for animal that I've seen before include "Man or beast" , "Possibly bear" , "Brutal, sensual" , "Source of meat" , "Brutish sort" .)