Neither number's right (3)
I believe the answer is:
nor
'neither' is the definition.
(eg in the phrase 'neither do I')
'number's right' is the wordplay.
'number' becomes 'no' (abbreviation).
'right' becomes 'r' (common abbreviation).
'no'+'r'='NOR'
(Other definitions for nor that I've seen before include "Negative conjunction" , "And not either" , "logic circuit" , "It's also not" , "Neither one ... the other" .)