A length of sawn timber (5)
I believe the answer is:
plank
'a length of sawn timber' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for plank that I've seen before include "Prepared piece of wood" , "Proverbially thick piece of wood" , "Length of wood" , "item in political party's programme" , "Long piece of timber" .)