Article in good wood
I believe the answer is:
oak
'wood' is the definition.
(oak is a kind of wood)
'article in good' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'in' means one lot of letters goes inside another.
'good' becomes 'ok'.
'a' inserted within 'ok' is 'OAK'.
(Other definitions for oak that I've seen before include "Common deciduous tree" , "Acorn tree" , "Building timber" , "Acorn-bearing tree" , "door at Oxford" .)