Fish that hurt dog (6)
I believe the answer is:
shadow
'dog' is the definition.
(thesaurus)
'fish that hurt' is the wordplay.
'fish' becomes 'shad' (shad is a kind of fish**).
'that hurt' becomes 'ow' (I've seen this in other clues).
'shad'+'ow'='SHADOW'
(Other definitions for shadow that I've seen before include "opposing counterpart" , "Dark area from which light is blocked" , "Dark area - referring to parliamentary opposition" , "Darker part" , "Trail" .)