At first some take up commercial construction of building material (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Initial letters (at first) of S [ome] T [ake] U [p] C [ommercial] C [onstruction] O [f]. Traditional building material, used either as a surface coating or between strips of timber in “lath and plaster” construction.