Type of sugar in cola set to change (7)
I believe the answer is:
lactose
'type of sugar' is the definition.
'cola set to change' is the wordplay.
'to change' indicates anagramming the letters.
'cola'+'set'='colaset'
'colaset' anagrammed gives 'LACTOSE'.
'in' acts as a link.
(Other definitions for lactose that I've seen before include "Part of milk" , "milky sweetener" , "Constituent of milk" , "A sugar present in milk" , "White substance in milk" .)