Insane character possessed substance according to Spooner (3,6)
I believe the answer is:
mad hatter
'possessed substance according to spooner' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'insane character' is the wordplay.
'insane' becomes 'mad' (synonyms).
'character' becomes 'hatter' (I can't explain this - if you can you should believe this answer much more).
'mad'+'hatter'='MAD HATTER'
Can you help me to learn more?
(Other definitions for mad hatter that I've seen before include "Participant in party in Wonderland" , "one forever taking tea" , "Carroll character" , "attending a literary tea party" , "Bizarre party host" .)