Regularly stained construction material (4)
I believe the answer is:
sand
'construction material' is the definition.
'sand' can be an answer for 'material' (sand is a kind of material). I'm not sure about the 'construction' bit.
'regularly stained' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'stained' are 'SAND'.
Can you help me to learn more?
(Other definitions for sand that I've seen before include "Beach grit" , "Yellow-brown colour" , "Dune constituent" , "yellowish-brown" , "Desert material" .)