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' is the 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" .)