Heavy waterproof cover (9)
I believe the answer is:
tarpaulin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">heavy waterproof cover</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 tarpaulin that I've seen before include "protective cover" , "seaman rarely required" , "Waterproof canvas used as covering" , "Strong waterproof sheet" , "canvas given treatment?" .)