Dwarf fish from the south (3)
I believe the answer is:
doc
'dwarf' is the definition.
(dwarf in Disney's Snow White and the Seven Dwarfs)
'fish from the south' is the wordplay.
'fish' becomes 'cod' (type of fish).
'from the south' says the letters should be written backwards (in a down clue, letters go up).
'cod' reversed gives 'DOC'.
(Other definitions for doc that I've seen before include "Quack" , "scaled-down version of GP" , "Medical practitioner (abbr)" , "Medic (colloq.)" , "Holliday (in 17 24)" .)