Hunk's hat is sweet (10)
I believe the answer is:
gobstopper
'is sweet' is the definition.
(type of large sweet)
'hunk's hat' is the wordplay.
'hunk's' becomes 'gobs' (I can't explain this - if you can you should give a lot more credence to this answer).
'hat' becomes 'topper' (topper is a kind of hat).
'gobs'+'topper'='GOBSTOPPER'
Can you help me to learn more?
(Other definitions for gobstopper that I've seen before include "Big sweet" , "Large, hard spherical sweet" , "Large, hard sweet to suck" , "Hard round sweet" .)