Roofing material that's least troublesome (5)
I believe the answer is:
slate
'roofing material' is the definition.
(I know that slate is a type of roofing material)
'least troublesome' is the wordplay.
'troublesome' is an anagram indicator.
'least' is an anagram of 'SLATE'.
'that's' acts as a link.
(Other definitions for slate that I've seen before include "Scold; hard rock" , "Bluish-grey colour" , "Roofing plate" , "Criticise - outdated school equipment" , "Metamorphic rock that can be split into thin layers" .)