Fluffy clouds (6)
I believe the answer is:
cumuli
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fluffy clouds</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for cumuli that I've seen before include "Clouds of round masses heaped on horizontal base" , "Rounded clouds with flat bases" , "Globular clouds" , "Fluffy clouds" , "Plural of clouds heaped on horizontal bases" .)