The king’s evil son abused our calf (8)
I believe the answer is:
scrofula
'the king's evil' is the definition.
(synonyms)
'son abused our calf' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'abused' indicates an anagram.
'our'+'calf'='ourcalf'
'ourcalf' with letters rearranged gives 'crofula'.
's'+'crofula'='SCROFULA'
(Other definitions for scrofula that I've seen before include "Old disease" , "A disease of flour sac causing glandular swellings" , "King's evil - foul scar (anag)" .)