Disease ravaging Serbia? (6)
I believe the answer is:
rabies
'disease' is the definition.
(rabies is a kind of disease)
'ravaging serbia?' is the wordplay.
'ravaging' is an anagram indicator.
'serbia' with letters rearranged gives 'RABIES'.
(Other definitions for rabies that I've seen before include "Killer disease" , "Fatal virus disease affecting dogs" , "Disease carried by mad dog" , "illness (like 13)" , "Disease associated with mad dogs" .)