Cereal succeeded in upsetting part of digestive system (6)
I believe the answer is:
muesli
'cereal' is the definition.
(I've seen this before)
'succeeded in upsetting part of digestive system' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'in' indicates putting letters inside.
'upsetting' says the letters should be written backwards.
'part of digestive system' becomes 'ileum' (I've seen this before).
'ileum' back-to-front is 'mueli'.
's' put inside 'mueli' is 'MUESLI'.
(Other definitions for muesli that I've seen before include "healthy breakfast?" , "Breakfast food of cereals, fruits, nuts etc" , "Mixture of crushed cereals. nuts and fruit" , "Breakfast dish with cereals, nuts, fruit etc" , "Mixture of cereals etc" .)