Discarded fish in to be cooked (8)
I believe the answer is:
obsolete
'discarded' is the definition.
(I've seen this in another clue)
'fish in to be cooked' is the wordplay.
'fish' becomes 'sole' (sole is a kind of fish).
'in' indicates putting letters inside.
'cooked' indicates an anagram (letters cooked into a new form).
'to'+'be'='tobe'
'tobe' is an anagram of 'obte'.
'sole' going into 'obte' is 'OBSOLETE'.
(Other definitions for obsolete that I've seen before include "Discontinued" , "Obese lot may be out of date" , "Old hat" , "Pass" , "No longer current" .)