Doubtless one isn't (9)
I believe the answer is:
uncertain
'doubtless' is the definition.
The answer and definition are not the same part of speech.
'one isn't' is the wordplay.
'one' becomes 'un' (northern English dialect form of 'one').
'isn't' becomes 'certain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'un'+'certain'='UNCERTAIN'
Can you help me to learn more?
(Other definitions for uncertain that I've seen before include "Unsure" , "In doubt" , "up in the air" , "Not completely confident or sure of" , "Not convinced" .)