Carpentry groove (4)
I believe the answer is:
dado
'carpentry groove' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dado that I've seen before include "Wood border running round room walls" , "Decorated lower part of an interior wall" , "Panelled lower part of a wall" , "stretch of wall" , "strip running round room" .)