Roofing material poisonous thing? Stop! (7)
I believe the answer is:
asphalt
'roofing material' is the definition.
(I've seen this before)
'poisonous thing? stop' is the wordplay.
'poisonous thing?' becomes 'asp' (poisonous snake).
'stop' becomes 'halt' (synonyms).
'asp'+'halt'='ASPHALT'
(Other definitions for asphalt that I've seen before include "Road surface material" , "Bituminous substance used for roofs and roads" , "Bituminous substance used in building" , "Paving material" , "petroleum residue" .)