Flawless atrium evenly coated with gemstone (7)
I believe the answer is:
optimal
'flawless' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'atrium evenly coated with gemstone' is the wordplay.
'evenly' means one should take alternating letters (the letters in even-numbered positions).
'coated with' indicates putting letters inside.
'gemstone' becomes 'opal' (opal is a kind of gemstone).
The alternating letters of 'atrium' are 'tim'.
'tim' put into 'opal' is 'OPTIMAL'.
Can you help me to learn more?
(Other definitions for optimal that I've seen before include "Plot aim (anag) - best" , "Best or most favourable" , "In the best condition" , "couldn't be better" , "Ideal" .)