Book featured dietary component (4)
I believe the answer is:
bran
'dietary component' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'book featured' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'featured' becomes 'ran' (I can't explain this - if you can you should give a lot more credence to this answer).
'b'+'ran'='BRAN'
Can you help me to learn more?
(Other definitions for bran that I've seen before include "Husks of grain sifted from the flour" , "Food from the husks of cereal grains" , "Food for horses in barn" , "Health food" , "DOG" .)