Handbill that can be used to keep the rain out (8)
I believe the answer is:
flysheet
I believe this clue is a double definition.
'handbill' is the first definition.
(a flysheet is a handbill or leaflet)
'that can be used to keep the rain out' is the second definition.
(sheet covering a tent)
(Other definitions for flysheet that I've seen before include "Handbill - extra protection for a tent" , "extra protection when camping" .)