Brown songbird raises cuckoo inside middle of reserve (9)
I believe the answer is:
chocolate
'brown' is the definition.
(chocolate is a kind of brown)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for chocolate that I've seen before include "Delicious food of roasted ground cacao beans" , "The tasty treat turns out to be a cool cheat" , "Deep brown colour" , "Coca hotel is a treat" , "The tasty substance could be a cool cheat 99)" .)