Roof straw (6)
I believe the answer is:
thatch
'roof straw' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thatch that I've seen before include "Rural roofing" , "Roofing material - thick hair" , "Reed or straw roofing" , "Alternative to lead roof" , "Plant stalks as roofing material" .)