One who cures fish from the south (3)
I believe the answer is:
doc
'one who cures' is the definition.
(short for doctor)
'fish from the south' is the wordplay.
'fish' becomes 'cod' (type of fish).
'from the south' is a reversal indicator (in a down clue, letters go up).
'cod' in reverse letter order is 'DOC'.
(Other definitions for doc that I've seen before include "GP (informal)" , "Medic, in short" , "scaled-down version of GP" , "Medic (colloq.)" , "Disney dwarf" .)