Foggy — incomprehensible (6)
I believe the answer is:
opaque
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">foggy</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">incomprehensible</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for opaque that I've seen before include "Impervious to light - hard to understand" , "Without lustre" , "Cloudy, non-transparent" , "Incomprehensible, impenetrable" , "Mysterious" .)