Sugar drop about to take effect (7)
I believe the answer is:
lactose
'sugar' is the definition.
(I know that lactose is a type of disaccharide)
'drop about to take effect' is the wordplay.
'drop' becomes 'lose' (I've seen this in another clue).
'about' indicates putting letters inside.
'to take effect' becomes 'act' (I've seen this in another clue).
'lose' placed around 'act' is 'LACTOSE'.
(Other definitions for lactose that I've seen before include "White substance in milk" , "Constituent of milk" , "A sugar present in milk" , "Sugar found in milk" , "milky sweetener" .)