Cruel disorder in the stomach (5)
I believe the answer is:
ulcer
'the stomach' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'cruel disorder' is the wordplay.
'disorder' indicates an anagram (a disordered version of the letters).
'cruel' with letters rearranged gives 'ULCER'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for ulcer that I've seen before include "Spot of bother" , "Inflammatory internal or external lesion" , "cause of suffering" , "Inflamed lesion" , "Cruel kind of open sore" .)