Like a perfectly upright timber (5)
I believe the answer is:
ideal
'like' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'a perfectly upright timber' is the wordplay.
'a perfectly upright' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'timber' becomes 'deal' (deal is a kind of timber).
'i'+'deal'='IDEAL'
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "fine" , "(the best one can get)" , "Benchmark" , "To which one aspires" , "Ailed (anag)" .)