Brian's demented, one thinks (5)
I believe the answer is:
brain
'brian's demented one thinks' is the definition.
I can't judge whether this definition defines the answer.
'brian's demented' is the wordplay.
'demented' indicates an anagram.
'brian' anagrammed gives 'BRAIN'.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Control centre of nervous system" , "Clever type" , "It's in the skull" , "Thinking thing" , "The grey matter" .)