Anger over a disease (7)
I believe the answer is:
cholera
'disease' is the definition.
(cholera is a kind of health problem)
'anger over a' is the wordplay.
'anger' becomes 'choler' (synonyms).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'choler'+'a'='CHOLERA'
(Other definitions for cholera that I've seen before include "bacterial disease" , "Solemn promise" , "Disease caught from contaminated water" , "Deadly, infectious disease" , "Acute intestinal infection from contaminated water or food" .)