Invalid, not binding (4)
I believe the answer is:
null
I believe this clue is a double definition.
'invalid' is the first definition.
(similar in meaning)
'not binding' is the second definition.
(I've seen this in another clue)
(Other definitions for null that I've seen before include "Inoperative" , "Having the value zero -- void" , "Legally void" , "Invalid; empty" , "Without legal force" .)