Not even adverbial (5)
I believe the answer is:
oddly
'not even adverbial' is the definition.
The definition and answer are not the same part of speech.
'not even adverbial' is the wordplay.
'not even' becomes 'odd' ('odd' is the opposite to 'even').
'adverbial' becomes 'ly' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'odd'+'ly'='ODDLY'
Can you help me to learn more?
(Other definitions for oddly that I've seen before include "Weirdly" , "Strangely, curiously" , "In a peculiar way" , "in a curious way" , "In a strange manner" .)