Strip of wood (4)
'strip of wood' 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 "Strip of wood used as a base for plastering" , "Plaster-supporting wood strip" , "Wood underlying plaster" , "Narrow thin strip of wood" , "Halt bearing strip of wood" .)
(Other definitions for plank that I've seen before include "Length of wood" , "Strip of timber" , "Long piece of timber" , "Wooden board" , "Long piece of wood" .)