Sugar upsetting a Celt so (7)
I believe the answer is:
lactose
'sugar' is the definition.
(lactose is a kind of sugar)
'upsetting a celt so' is the wordplay.
'upsetting' indicates an anagram.
'a'+'celt'+'so'='aceltso'
'aceltso' is an anagram of 'LACTOSE'.
(Other definitions for lactose that I've seen before include "White substance in milk" , "A sugar present in milk" , "milky sweetener" , "Part of milk" , "Sugar found in milk" .)