Of no effect (4)
I believe the answer is:
null
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of no effect</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for null that I've seen before include "Without force, validity" , "Useless - nothing" , "Invalid; empty" , "Invalid, void" , "Not valid" .)