One form of wood that's perfect (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'one form of wood' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'wood' could be 'deal' (deal is a kind of wood) and 'deal' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "without a flaw" , "Utopian; yardstick" , "Model of ethical behaviour" , "Model of perfection" , "To which one aspires" .)