Tent part (11)
(Other definitions for groundsheet that I've seen before include "camping equipment" , "Camper's protection from damp" , "campers need it" , "Waterproof base for tent" , "protective cover" .)
'tent part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flap that I've seen before include "Minor panic; tab" , "Panic; fold" , "Move wings - panic!" , "Agitated state; tab" , "Adjustable part of wing - panic" .)