An article in sound timber (3)
I believe the answer is:
oak
'sound timber' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'an article in' is the wordplay.
'an' becomes 'a'.
'article' becomes 'ok' (I can't justify this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'a' placed within 'ok' is 'OAK'.
Can you help me to learn more?
(Other definitions for oak that I've seen before include "Building timber" , "From little acorns does it grow" , "The King of the Forest" , "Hardwood tree" , "Nose of certain wines" .)