Operate inside to go down to get milk-sugar (7)
I believe the answer is:
lactose
'sugar' is the definition.
(I know that lactose is a type of disaccharide)
'operate inside to go down' is the wordplay.
'operate' becomes 'act' (I've seen this in another clue).
'inside' is an insertion indicator.
'to go down' becomes 'lose' (I've seen this in another clue).
'act' going into 'lose' is 'LACTOSE'.
'to get milk' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lactose that I've seen before include "milky sweetener" , "Constituent of milk" , "Sugar found in milk" , "A sugar present in milk" , "Part of milk" .)