Sound good before illness returns (4)
I believe the answer is:
Here is my best explanation:
'sound' is the definition.
'good before illness returns' is the subsidiary indication.
'good' becomes 'g' (abbreviation).
'before' is a charade indicator (letters next to each other).
'illness' becomes 'flu' (flu is a kind of illness).
'returns' shows that the letters should be reversed in order.
'flu' written backwards gives 'ulf'.
(Other definitions for gulf that I've seen before include "Deep sea inlet, as between Iran and Saudi Arabia", "Large inlet of the sea", "Unbridgeable disparity", "Chasm, large gap", "Abyss", "Deep wide chasm", "Wide gap; arm of sea", "Rift", "Wide separation" and "Wide difference of opinion".)