So curse doctor! Sugar! (7)
I believe the answer is:
sucrose
'sugar' is the definition.
(I know that sucrose is a type of disaccharide)
'so curse doctor' is the wordplay.
'doctor' indicates anagramming the letters.
'so'+'curse'='socurse'
'socurse' with letters rearranged gives 'SUCROSE'.
(Other definitions for sucrose that I've seen before include "Sugar's required" , "Chief component of cane or beet sugar" , "Basis of cane sugar" , "Sweet substance" , "Main part of cane sugar" .)