Part of speech Eton possibly has not sanctioned (8)
I believe the answer is:
verboten
'not sanctioned' is the definition.
(I've seen this before)
'part of speech eton possibly' is the wordplay.
'part of speech' becomes 'verb' (verb is a kind of part of speech in grammar).
'possibly' is an anagram indicator.
'eton' with letters rearranged gives 'oten'.
'verb'+'oten'='VERBOTEN'
'has' acts as a link.
(Other definitions for verboten that I've seen before include "Banned (German)" , "Not sanctioned" , "Strictly not allowed" , "Forbidden (German)" , "Prohibited" .)