Sugar cost changing in brewing of ale (7)
'sugar' is the definition.
(lactose is a kind of sugar)
'cost changing in brewing of ale' is the wordplay.
'changing' indicates an anagram.
'in' is an insertion indicator.
'brewing of' indicates anagramming the letters (I've seen 'brew' mean this).
'cost' anagrammed gives 'ctos'.
'ale' anagrammed gives 'lae'.
'ctos' going into 'lae' is 'LACTOSE'.
(Other definitions for lactose that I've seen before include "milky sweetener", "Constituent of milk", "A sugar present in milk", "Sugar found in milk", "Part of milk".)