Neither number is right! (3)
I believe the answer is:
nor
'neither' is the definition.
(I've seen this before)
'number is right' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'is right' becomes 'r' (common abbreviation).
'no'+'r'='NOR'
(Other definitions for nor that I've seen before include "logic circuit" , "Common conjunction, goes with neither" , "Negative word" , "And not; Norman (abbr.)" , "Neither one ... the other" .)