Medic acts badly about onset of illness that's severe (7)
I believe the answer is:
drastic
'severe' is the definition.
(thesaurus)
'medic acts badly about onset of illness' is the wordplay.
'medic' becomes 'dr'.
'badly' indicates an anagram.
'about' means one lot of letters goes inside another.
'onset of' says to take the initial letters.
The initial letter of 'illness' is 'i'.
'acts' with letters rearranged gives 'astc'.
'astc' placed around 'i' is 'astic'.
'dr'+'astic'='DRASTIC'
'that's' acts as a link.
(Other definitions for drastic that I've seen before include "Extreme - violent" , "Draconian" , "Thoroughgoing" , "Radical, extreme" , "'Severe, extreme (7)'" .)