A sweet nothing? Not exactly! (6)
I believe the answer is:
nougat
'a sweet' is the definition.
(nougat is a kind of sweet)
'nothing? not exactly' is the wordplay.
I cannot really understand how this works, but
'nothing?' could be 'no' (similar in meaning) and 'no' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nougat that I've seen before include "Chewy sweet made of egg-white, nuts and sugar" , "Honeyed sweet" , "Honey/nuts sweet" , "Hard, chewy sweet" , "Chewy confection with nuts, fruit, etc" .)