Cirrus or Cumulus (5)
I believe the answer is:
cloud
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">cirrus</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">cumulus</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for cloud that I've seen before include "Make less clear" , "Nebula" , "Mass of vapour visible in the air" , "Mass of water droplets" , "Visible vapour in sky" .)