Brown is fattening (9)
I believe the answer is:
chocolate
'brown' is the definition.
(I know that chocolate is a type of brown)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for chocolate that I've seen before include "Addictive substance from beans" , "The tasty substance could be a cool cheat 99)" , "Coca hotel is a treat" , "Deep brown colour" , "The tasty treat could turn out to be a cool cheat" .)