Befuddle (5)
(Other definitions for nonplus that I've seen before include "Dumbfound" , "disadvantageous or neutral, presumably" , "Baffle" , "Utterly confuse" , "Cause dismay" .)
'befuddle' is the definition.
(I know that befuddle can be written as confuse)
This is all the clue.
(Other definitions for confuse that I've seen before include "Disconcert, befuddle" , "Bewilder" , "Puzzle" , "Muddle, mix up" , "Perplex" .)
'befuddle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for addle that I've seen before include "becomes muddled" , "Befuddle" , "Like a bad egg" , "Mix up or confuse" , "Make confused - go bad" .)