If not, except when (6)
I believe the answer is:
unless
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">if not</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">except when</span>' is the second definition.<br/ ><span class="explanation-format__explanation">('except when' can mean 'unless')</span><br/ ><br/ ></span>
(Other definitions for unless that I've seen before include "Except on the condition that" , "Otherwise than if" , "Though" , "Except, save" , "Conjunction meaning except when" .)