Try hollow brimless hat that’s sweet (10)
I believe the answer is:
gobstopper
'that's sweet' is the definition.
(type of large sweet)
'try hollow brimless hat' is the wordplay.
'try' becomes 'go' (both can mean an attempt).
'hollow' means to remove the middle letters.
'hat' becomes 'topper' (topper is a kind of hat).
'brimless' with its middle taken out is 'bs'.
'go'+'bs'+'topper'='GOBSTOPPER'
(Other definitions for gobstopper that I've seen before include "Big sweet" , "Large, hard sweet to suck" , "Large, hard spherical sweet" , "Hard round sweet" .)