Partner of "void" (4)
I believe the answer is:
null
'partner of void' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for null that I've seen before include "... and void, lacking any legal or binding force" , "Invalid, void" , "Not valid" , "Lacking any legally binding force" , "Useless - nothing" .)