Sheet of low cloud (7)
I believe the answer is:
stratus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sheet of low cloud</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for stratus that I've seen before include "Could carry rain" , "Large dark low cloud" , "Cloud as a grey sheet" , "Grey cover" , "Sheet of low cloud" .)