Slight change until accepting source of sickness (6)
I believe the answer is:
insult
'slight' is the definition.
(thesaurus)
'change until accepting source of sickness' is the wordplay.
'change' is an anagram indicator.
'accepting' indicates putting letters inside.
'source of' indicates taking the first letters.
The first letter of 'sickness' is 's'.
'until' anagrammed gives 'inult'.
'inult' going around 's' is 'INSULT'.
(Other definitions for insult that I've seen before include "Demeaning remark" , "Affront, offend" , "Treat with abuse" , "Treat disrespectfully" , "Abusive remark" .)