Rich food source of tummy upset (7)
I believe the answer is:
truffle
'rich food' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for truffle that I've seen before include "Rich food" , "Sweet - edible fungus growing underground" , "Valued culinary fungus" , "Witty remark" , "Underground delicacy" .)