Sweet, say, particularly covered in fluff (12)
I believe the answer is:
butterscotch
'sweet' is the definition.
(butterscotch is a kind of sweet)
'say particularly covered in fluff' is the wordplay.
'say' becomes 'utter' (to utter something is to speak or say it).
'particularly covered' becomes 'sc' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'fluff' becomes 'botch' (synonyms).
'utter'+'sc'='uttersc'
'uttersc' placed into 'botch' is 'BUTTERSCOTCH'.
Can you help me to learn more?
(Other definitions for butterscotch that I've seen before include "Brittle, yellow-brown sweet" , "Kind of toffee" , "Type of toffee" , "Hard brittle toffee" , "confectionery" .)