Gloomy about new building material (4)
I believe the answer is:
sand
'material' is the definition.
(sand is a kind of material)
'gloomy about new' is the wordplay.
'gloomy' becomes 'sad' (similar in meaning).
'about' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'sad' going around 'n' is 'SAND'.
'building' acts as a link.
(Other definitions for sand that I've seen before include "Seaside castle-building material" , "Result of rock erosion" , "lover of Chopin" , "Sort of paper, piper, castle or martin" , "Yellow-brown colour" .)