Plaster base (4)
I believe the answer is:
lath
'plaster base' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lath that I've seen before include "Plaster-supporting wood strip" , "Narrow strip of wood used as backing for plasterwork" , "Thin wood strip supporting plaster" , "Narrow thin strip of wood" , "Thin strip of wood used in plastering" .)