Minus? Completely baffle (7)
I believe the answer is:
nonplus
'baffle' is the definition.
(I know that baffle can be written as nonplus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nonplus that I've seen before include "Put at a loss; confuse" , "Perplex completely with negative and positive" , "disadvantageous or neutral, presumably" , "Get flummoxed" , "Baffle" .)