Forged art, say, is wrong (6)
I believe the answer is:
astray
'is wrong' is the definition.
(going astray is going wrong)
'forged art say' is the wordplay.
'forged' indicates anagramming the letters.
'art'+'say'='artsay'
'artsay' anagrammed gives 'ASTRAY'.
(Other definitions for astray that I've seen before include "Wandering, off course" , "Philip -, English twentieth century poet" , "Wandering lost" , "in the wrong place" , "(Gone) off course" .)