As nice as important constituent of cheese (6)
I believe the answer is:
casein
'constituent of cheese' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'as nice as important' is the wordplay.
'as nice' can be anagrammed to 'CASEIN'.
But, I am not sure how this is indicated.
Can you help me to learn more?
(Other definitions for casein that I've seen before include "What's to be found in milk" , "something for cheese-making" , "It helps to make cheese" , "Protein in milk and cheese" .)