Fungus in chocolate (7)
I believe the answer is:
truffle
I believe this is a double definition.
'fungus' is the first definition.
(I know that truffle is a type of fungus)
'chocolate' is the second definition.
(I know that truffle is a type of chocolate)
'in' acts as a link.
(Other definitions for truffle that I've seen before include "Sweet - edible fungus growing underground" , "Soft chocolate; fungus" , "Witty remark" , "Edible fungus -- chocolate sweet" , "Fungoid delicacy" .)