Dwarf fish returned (3)
I believe the answer is:
doc
'dwarf' is the definition.
(one of the seven)
'fish returned' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'returned' says the letters should be written in reverse.
'cod' written backwards gives 'DOC'.
(Other definitions for doc that I've seen before include "Holliday (in 17 24)" , "Main dish" , "One of the seven dwarfs" , "Physician (colloq.)" , "Medical practitioner (abbr)" .)