Narrow board (4)
'narrow board' 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 "Wood underlying plaster" , "Plaster-supporting wood strip" , "Narrow strip of wood used as backing for plasterwork" , "Narrow thin strip of wood" , "Halt bearing strip of wood" .)
(Other definitions for slat that I've seen before include "Part of a Venetian blind" , "Thin strip" .)