Sore bullock oddly dropped on back (5)
I believe the answer is:
ulcer
'sore' is the definition.
(thesaurus)
'bullock oddly dropped on back' is the wordplay.
'oddly' indicates alternate letters.
'dropped on back' becomes 'er' (I can't justify this - if you can you should believe this answer much more).
The alternating letters of 'bullock' are 'ulc'.
'ulc'+'er'='ULCER'
Can you help me to learn more?
(Other definitions for ulcer that I've seen before include "Cruel sort of open sore" , "Cruel kind of bodily sore" , "Cruel form of stomach trouble?" , "Inflammatory internal or external lesion" , "Stomach or mouth sore" .)