Doctor locates source of food intolerance? (7)
I believe the answer is:
lactose
'source of food intolerance?' is the definition.
(as in lactose intolerance)
'doctor locates' is the wordplay.
'doctor' indicates anagramming the letters.
'locates' with letters rearranged gives 'LACTOSE'.
(Other definitions for lactose that I've seen before include "Part of milk" , "A sugar present in milk" , "White substance in milk" , "Sugar found in milk" , "Constituent of milk" .)