Type of sugar (6)
'type of sugar' is the definition.
This is all the clue.
(Other definitions for fructose that I've seen before include "A simple sugar, in honey for instance" , "Kind of sugar" , "Sweetener" , "It's sweet" , "Fruit-sugar" .)
(Other definitions for caster that I've seen before include "One who throws something" , "Crates with kind of fine sugar" , "Small swivelling wheel on furniture" , "Swivelling wheel (E, not O)" , "Founder; type of 12 Ac" .)