Note their confusion, both one and the other (6)
I believe the answer is:
either
'both one and the other' is the definition.
I can't tell whether this defines the answer.
'note their confusion' is the wordplay.
'note' becomes 'e' (musical note).
'confusion' is an anagram indicator.
'their' with letters rearranged gives 'ither'.
'e'+'ither'='EITHER'
Can you help me to learn more?
(Other definitions for either that I've seen before include "could be one of two" , "Each of two; moreover" , "No matter which (of two)" , "One of two alternatives" , "Choice of two" .)