Ailing, sick (6)
I believe the answer is:
unwell
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">ailing</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that ailing can be written as unwell)</span><br/ ><br/ >'<span class="explanation-format__original">sick</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for unwell that I've seen before include "Poorly" , "Off-colour" , "Indisposed" , "Sickly, low" , "Ill, not in good health" .)