The fish - a portion of haddock - is sent back (3)
I believe the answer is:
cod
'the fish' is the definition.
(type of fish)
'a portion of haddock is sent back' is the wordplay.
'a portion of' says the answer is hidden in the clue.
'is sent back' is a reversal indicator.
'doc' is hidden within 'haddock'.
'doc' back-to-front is 'COD'.
(Other definitions for cod that I've seen before include "method of payment" , "A foolish sort of a fish" , "Hoax (sl.); fish" , "A fool of a fish" , "Fish noted for liver oil" .)