Standard of perfection for one kind of timber (5)
I believe the answer is:
ideal
'standard of perfection' is the definition.
(I've seen this before)
'one kind of timber' is the wordplay.
'one' becomes 'I' (Roman numeral).
'kind of timber' becomes 'deal'.
'i'+'deal'='IDEAL'
'for' acts as a link.
(Other definitions for ideal that I've seen before include "Quite perfect, can't be improved on" , "fine" , "just the job" , "Model of excellence (5)." , "Perfectly suited" .)