Having no significance (4)
'having no significance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for meaningless that I've seen before include "Empty" , "Without significance" , "Men's angle is (anag) -- nonsensical" , "having no worthwhile quality" , "Futile" .)
(Other definitions for null that I've seen before include "Useless - nothing" , "Immaterial and void" , "Legally void" , "Inoperative" , "Invalid, void" .)