Trout cooked by radical on the rack (8)
I believe the answer is:
tortured
'on the rack' is the definition.
(thesaurus)
'trout cooked by radical' is the wordplay.
'cooked' indicates an anagram (letters cooked into a new form).
'by' is a charade indicator (letters next to each other).
'radical' becomes 'red' (red can informally mean communist).
'trout' anagrammed gives 'tortu'.
'tortu'+'red'='TORTURED'
(Other definitions for tortured that I've seen before include "Tormented emotionally or mentally" , "Subjected to painful punishment" , "Subjected to great pain" , "Made to suffer greatly" , "Subjected to great suffering" .)