Builder's backing (4)
I believe the answer is:
lath
'builder's backing' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lath that I've seen before include "Narrow strip of wood as used in trellis" , "Thin wood strip supporting plaster" , "Wood underlying plaster" , "Strip of wood used with plaster" , "Strip of wood, backing for plaster say" .)