Improved by one type of wood (5)
I believe the answer is:
ebony
'type of wood' is the definition.
(I know that ebony is a type of wood)
'improved by one' is the wordplay.
'improved' indicates an anagram.
'by'+'one'='byone'
'byone' is an anagram of 'EBONY'.
(Other definitions for ebony that I've seen before include "Very dark brown" , "'Hard, dark wood (5)'" , "Hard block tropical wood" , "Tree having black wood" , "Dark hardwood" .)