Red mullet's tail by itself, cooked (7)
I believe the answer is:
leftist
'red' is the definition.
(I've seen this in another clue)
'mullet's tail by itself cooked' is the wordplay.
'tail' says to take the final letters.
'by' means one lot of letters go next to another.
'cooked' indicates anagramming the letters (letters cooked into a new form).
The final letter of 'mullet' is 't'.
'itself' is an anagram of 'leftis'.
't' after 'leftis' is 'LEFTIST'.
(Other definitions for leftist that I've seen before include "Red" , "Labourite" , "Supporter of socialism" , "Radical politician" , "Labour Party member" .)