One form of timber that's perfect (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'one form of timber' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'timber' could be 'deal' (deal is a kind of timber) and 'deal' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "Paragon" , "Cutting/stabbing weapons" , "Coal-mining waste" , "Perfect; a standard of perfection" , "immaculate" .)