Awfully nice as a protein (6)
I believe the answer is:
casein
'a protein' is the definition.
(casein is a kind of protein)
'awfully nice as' is the wordplay.
'awfully' indicates an anagram.
'nice'+'as'='niceas'
'niceas' with letters rearranged gives 'CASEIN'.
(Other definitions for casein that I've seen before include "something for cheese-making" , "It helps to make cheese" , "Protein in milk and cheese" , "What's to be found in milk" .)