Fish from harbour mostly thrown back (3)
I believe the answer is:
cod
'fish' is the definition.
(type of fish)
'harbour mostly thrown back' is the wordplay.
'harbour' becomes 'dock' (I've seen this before).
'mostly' means to remove the last letter (most but not all of the word).
'thrown back' is a reversal indicator.
'dock' with its last letter taken off is 'doc'.
'doc' written backwards gives 'COD'.
'from' acts as a link.
(Other definitions for cod that I've seen before include "mock" , "faked" , "Edible fish" , "A fool of a fish" , "Initially, pay when you get the goods" .)