A codeword unusually used for timber (9)
I believe the answer is:
cedarwood
'for timber' is the definition.
(I've seen this before)
'a codeword unusually' is the wordplay.
'unusually' is an anagram indicator.
'a'+'codeword'='acodeword'
'acodeword' with letters rearranged gives 'CEDARWOOD'.
'used' is the link.
(Other definitions for cedarwood that I've seen before include "furniture maker" , "'Sandalwood, ____ and 4 down 18 19' (Masefield)" , "it comes out of a tree" , "Hard, sweet-smelling timber" .)