Baleful sickness upset no-good individual (6)
I believe the answer is:
malign
'baleful' is the definition.
(thesaurus)
'sickness upset no-good individual' is the wordplay.
'sickness' becomes 'mal' (I've seen this before**).
'upset' is an anagram indicator.
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'individual' becomes 'i'.
'n'+'g'+'i'='ngi'
'ngi' is an anagram of 'ign'.
'mal'+'ign'='MALIGN'
(Other definitions for malign that I've seen before include "Evil-natured" , "Bad-mouth" , "Showing malice, say harmful things about" , "Laming, showing malice" , "Evil - cancerous" .)