Cruel sort of complaint (5)
I believe the answer is:
ulcer
'complaint' is the definition.
(a medical complaint)
'cruel sort' is the wordplay.
'sort' is an anagram indicator.
'cruel' with letters rearranged gives 'ULCER'.
'of' acts as a link.
(Other definitions for ulcer that I've seen before include "Cruel kind of bodily sore" , "Sore point" , "Inflammatory internal or external lesion" , "Spot of bother" , "Cruel sort of open sore" .)