High, wispy clouds (5)
I believe the answer is:
cirri
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">high wispy clouds</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cirri that I've seen before include "High wisps of clouds" , "Highest clouds" , "High altitude clouds forming 'mare's tails'" .)