Void's partner (4)
I believe the answer is:
null
'void's partner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for null that I've seen before include "Having the value zero -- void" , "Not valid" , "Invalid - not binding" , "Without value or consequence" , "... and void, lacking any legal or binding force" .)