One kind of timber is just right (5)
I believe the answer is:
ideal
'just right' 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'
'is' acts as a link.
(Other definitions for ideal that I've seen before include "Standard to aim for" , "immaculate" , "existing only in imagination" , "example" , "Exemplary" .)