Girl unwell after onset of jaundice (4)
I believe the answer is:
jill
'girl' is the definition.
'unwell after onset of jaundice' is the wordplay.
'unwell' becomes 'ill' ('ill' can be similar in meaning to 'unwell').
'after' is a charade indicator (letters next to each other).
'onset of' says to take the initial letters.
The initial letter of 'jaundice' is 'j'.
'ill' put after 'j' is 'JILL'.
(Other definitions for jill that I've seen before include "Female ferret" , "Jack's pet" , "Jack's nursery rhyme girl companion" .)