Part of the small intestine upset after ingesting sulphur in the breakfast mixture (6)
I believe the answer is:
muesli
'the breakfast mixture' is the definition.
(I've seen this before)
'part of the small intestine upset after' is the wordplay.
'part of' indicates putting letters inside.
'the small intestine' becomes 'ileum' (ileum is a kind of small intestine).
'upset' is a reversal indicator.
'after' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'ileum' written backwards gives 'mueli'.
'mueli' placed around 's' is 'MUESLI'.
'ingesting sulphur in' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for muesli that I've seen before include "Breakfast food of cereals, fruits, nuts etc" , "Rolled oats, nuts and fruit mixture" , "Cereal made with oats and nuts" , "Mixture of crushed cereals. nuts and fruit" , "Breakfast dish with cereals, nuts, fruit etc" .)