Sick inmate nursing ultimately fatal illness (7)
I believe the answer is:
ailment
'illness' is the definition.
(an ailment is an illness or health problem)
'sick inmate nursing ultimately fatal' is the wordplay.
'sick' is an anagram indicator.
'nursing' indicates putting letters inside.
'ultimately' says to take the final letters.
The last letter of 'fatal' is 'l'.
'inmate' is an anagram of 'aiment'.
'aiment' going around 'l' is 'AILMENT'.
(Other definitions for ailment that I've seen before include "infirmity" , "Slight but persistent illness" , "Tummy upset, for instance" , "Physical disorder" , "Malady" .)