Like lightning — don't eat (4)
I believe the answer is:
fast
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">like lightning</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">don't eat</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(fasting is not eating)</span><br/ ><br/ ></span>
(Other definitions for fast that I've seen before include "Firmly attached; rapid" , "Lent, perhaps" , "Period during which fare is reduced" , "Quickly; tight" , "not lento" .)