Neither number right (3)
I believe the answer is:
nor
'neither' is the definition.
(eg in the phrase 'neither do I')
'number 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 "Word paired with "neither"" , "And not either" , "Link word" , "Conjunction meaning 'and not'" , "In addition, not" .)