Sound good before illness returns (4)
I believe the answer is:
gulf
'sound' is the definition.
(I've seen this before)
'good before illness returns' is the wordplay.
'good' becomes 'g' (abbreviation).
'before' means one lot of letters go next to another.
'illness' becomes 'flu' (flu is a kind of health problem**).
'returns' says the letters should be written in reverse.
'flu' in reverse letter order is 'ulf'.
'g'+'ulf'='GULF'
(Other definitions for gulf that I've seen before include "Deep sea inlet, as between Iran and Saudi Arabia" , "Large inlet of the sea" , "Abyss" , "Chasm, large gap" , "Wide difference of opinion" .)