Roofing material’s to go first (4)
I believe the answer is:
lead
'go first' is the definition.
(to lead is to go first or go in front)
I don't understand how the remainder of the clue works.
(Other definitions for lead that I've seen before include "1 down, symbol Pb" , "Be in charge of; metal" , "First news item" , "Metal; clue" , "Metal; show the way" .)