Measure, including most of additional sugar (8)
I believe the answer is:
dextrose
'sugar' is the definition.
(I know that monosaccharide is a type of carbohydrate\nI know that dextroglucose is a type of glucose\nI know that glucose is a type of aldohexose\nTherefore, dextroglucose is a type of aldohexose\nI know that aldohexose is a type of aldose\nTherefore, dextroglucose is a type of aldose\nI know that aldose is a type of monosaccharide\nTherefore, dextroglucose is a type of monosaccharide\nTherefore, dextroglucose is a type of carbohydrate)
'measure including most of additional' is the wordplay.
'measure' becomes 'dose' (dose is a kind of measure).
'including' is an insertion indicator.
'most of' means to remove the last letter.
'additional' becomes 'extra' (I've seen this before).
'extra' with its last letter taken away is 'extr'.
'dose' going around 'extr' is 'DEXTROSE'.
(Other definitions for dextrose that I've seen before include "sweetener" , "Form of sugar, in honey, say" , "Form of glucose" , "sweet stuff" .)