During autumn, an unopened good book is perfect (10)
I believe the answer is:
infallible
'is perfect' is the definition.
(I've seen this before)
'during autumn an unopened good book' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'autumn' becomes 'fall' ('fall' can be a synonym of 'autumn').
'an unopened' means to remove the first letter.
'good book' becomes 'bible' ('bible' can be a synonym of 'good book').
'bible' with its initial letter taken off is 'ible'.
'in'+'fall'+'ible'='INFALLIBLE'
(Other definitions for infallible that I've seen before include "Unable to make a mistake" , "Incapable of error" , "faultless" , "Perfect" , "Always right" .)