Not ever (5)
I believe the answer is:
never
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not ever</span>' is the definition.<br/ ><span class="explanation-format__explanation">('never' is the opposite to 'ever')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for never that I've seen before include "On no account" , "when hell freezes over?" , "Surely not" , "'It's now or - - ' (14 song)" , "On no occasion" .)