Heavy waterproof cover (9)
I believe the answer is:
tarpaulin
'heavy waterproof cover' is the definition.
(I've seen this before)
This is all the clue.
(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?" .)