Eccentric witch, old (6)
I believe the answer is:
weirdo
'eccentric' is the definition.
(thesaurus)
'witch old' is the wordplay.
'witch' becomes 'weird' (I can't explain this - if you can you should give a lot more credence to this answer).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'weird'+'o'='WEIRDO'
Can you help me to learn more?
(Other definitions for weirdo that I've seen before include "Odd fish" , "Informally, an odd or eccentric person" , "queer fish" , "Slang for odd or eccentric person" , "strange character" .)