Score for nine is accepted by Countdown host: UNWITTING (4,3,5)
I believe the answer is:
none the wiser
'countdown host unwitting' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'score for nine is accepted' is the wordplay.
'score' becomes 'hewer' (I can't justify this - if you can you should believe this answer much more).
'for' means one lot of letters go next to another (I've seen this in other clues).
'nine' becomes 'nonet' (group of nine musicians).
'accepted' is an insertion indicator.
'hewer' put after 'nonet' is 'nonethewer'.
'nonethewer' enclosing 'is' is 'NONE THE WISER'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for none the wiser that I've seen before include "unlike Ancient Mariner's listener the next day?" , "in the dark still" , "Still baffled" , "Knowing no more than before" , "Still in ignorance" .)